Having taken these to Kegworth along with my Alpair/Tang Band actives and spent a weekend giving both systems a workout, it is pretty clear to me that the Tannoys will benefit from a bit of a re-work. The main issue is that, while they produce pretty decent bass for the size of cabinet (~25 Litres, ported), they don't achieve the same bass extension that I was getting from the A/TB system. So, the obvious thing to do is to beef up the bass end a bit. Cue Ebay - someone selling 4 Tannoy 8" woofers, taken from Tannoy DC2000 cabinets (the same drivers were also used in the DC3000), for not very many beer vouchers, so I pressed the button. The vague plan was to use two of these drivers either side to add a sub to each of the existing cabs - with luck, these might even be the right height to replace the wooden stands, to end up with a 3-way - cross over from the subs to the DC's 8" driver somewhere in the hundreds of Hz, then cross over from that to the DC's tweeter somewhere around 1.5KHz or higher.
Investigating the specs of the DC2000/DC3000 crossovers, it seems that the bass driver serves from 0 to 400 Hz, and the 8" cone of the DC driver serves from 0 to 2.3 kHz. (This in itself was interesting, as it suggests that my current crossover frequency of 1500 Hz is probably too low - confirmed by later measurements - see below). Both bass cones fire into the same cabinet volume, 31 litres sealed in the case of the DC2000 and 40 Litres slot ported for the DC3000. So this is a "2.1-way" configuration. Having done some measurements of the drivers using DATS, and some cabinet modelling in Basta!, neither of the Tannoy cabinets seems to be ideal for the configuration - Basta! comes up with suggested cabinet volumes of about 130 Litres for a ported cabinet and 65 Litres for a sealed cabinet with both drivers firing into the same volume. So the good news - there seems to be plenty of scope to do better than Tannoy did with their cabinets, but the bad news is that both solutions would result in larger boxes than I was hoping. Basta! also suggests that my existing cabs are rather less than optimal - 54 Litres for a ported cab, although if I just blocked off the port, it would be about right for a sealed cabinet (27 litres suggested). Interestingly, because the existing ported design is sub-optimal volume-wise, the predicted performance is very similar to the predicted sealed box performance - maybe a couple of dB's in it at the bottom end, which actually accords with my experimental results - blocking the port reduces the bass, but not by a great deal. This also stacks up with Tannoy's own suggestion - the DC drivers were originally destined for "in-wall" installation, and they suggest a minimum volume of 25 Litres.
In passing, messing around with Basta! demonstrated that, if the volume of the box is optimal for the "sealed box" solution, adding a port to the box seems to make only a marginal difference, but for a given driver, you get better bass performance out of an optimally-sized ported box than you do from an optimal sealed box. As the difference in optimal volume is roughly a factor of 2 between sealed and ported, the sealed box seems like the better choice if you are constrained on volume. Of course, one way around the box size issue is to use 2 drivers in an "Isobaric" configuration - that way, you get essentially the same performance as with a single driver, but in half the volume (see later)
I don't particularly want the final cabinets to be bigger than the existing footprint, so if I simply remove the current stands and insert boxes of same width/depth in their place, I have approximately a second 25 Litres to play with, which isn't a lot - but simply by adding a second single bass driver in a 25L sealed box is effectively going to give me an extra 6 dB's to play with at the bottom end, which is potentially useful. 25L is a shade less than Basta! indicates as the ideal volume (35L), but the reduction doesn't seem to have a massive effect on the FR curve. So solution #1 is kind of the "no brainer" solution - convert the existing ported cab to sealed, add a second sealed cab of very similar dimensions, and hey presto - 6 dB more at the bottom end, assuming that I go for the "2.5-way" style solution, where the DC's 8" driver serves both bass and mids. One advantage here is that the re-working of the woodwork is at a minimum. However, it also isn't terribly exciting and doesn't seem to make the best use of the fact that I have 4 extra drivers to play with, as it would only use 2 of them. I'm also not entirely convinced by the 2.5-way approach - it seems to me that it would be better to dedicate the DC's 8" driver to the mid-band and relieve it of duty below 400 Hz. However, definitely a possibility.
Building completely new cabs is clearly an option - that would allow me to put the DC driver in a separate compartment within the cab - say, 5 Litres - and use it just for the mids and highs. Modelled in Basta!, that doesn't look too bad; if I use a bass-to-mid crossover of 400 Hz, the small box actually gives a bit of a lift from 400 down to about 100 before rolling off quite sharply, so that could work well. Staying with the existing footprint, that would leave me about 50 Litres to play with for 2 bass drivers to fire into, which looks close enough to Basta's suggested 69 Litres (sealed), and might be a choice for solution #2, particularly if I stretched the footprint just a shade. However, as per the discussion above, simply sticking 2 of the bass drivers in a sealed box isn't going to get me much (if any) of an advantage over #1, other than separating mid from bass, which may be marginal.
Modelling 2 drivers in a ported box in Basta! suggests that improved bass performance could be achievable, but as said above, I would need 130 Litres - clearly a non-starter if I want to keep the cabs in the existing footprint. Even just one of the bass drivers firing into a ported box seems to need 65 Litres. The ported box solutions, as would be expected, start to roll off at a much lower frequency than the sealed box solutions, but the roll-off is a lot steeper; the two curves cross at around the mid-20's and at about 20 dB down, so although the sealed box is giving more below that frequency, it probably isn't going to be that audible. So solution #3 (existing DC driver in a small sealed compartment, then 2 bass drivers in a 130 Litre ported box) offers the possibility of some useful bass extension over solutions 1 and 2, but at the expense of a humungous cabinet - roughly twice the volume of 1 and 2. So it would be tempting, but not actually where I want to go.
The only other obvious combinations are to use pairs of the bass driver in an Isobaric configuration. As said above, the Basta! modelling says that doubling up the drivers allows you to achieve the same performance as a single driver, but with 1/2 of the cabinet volume. So with a single driver, Basta! says I need 65 Litres for a ported box and 35 for a sealed box; with 2 drivers in an Isobaric configuration, I can get the same performance out of 32.5 and 17.5 Litres, respectively. Interestingly, I should be able to build either style of Isobaric box such that it would fit the space vacated by the existing stands, particularly if I am prepared to reduce the volume of the existing cabs slightly. So solution #4 would be to add an isobaric sealed sub, and solution #5 would be to add an isobaric ported sub. Of these two, the ported sub (#5) is really the only one that is interesting, because there's no advantage in #4 over #1, and #4 is more difficult to build than #1.
So, the upshot of the above is that I seem to be talking myself into building a pair of Isobaric, ported subs (#5) to fit under the existing DC cabinets, possibly with a bit of surgery on the existing cabs to give me a tad more volume for the subs. Comparing the predicted performance in Basta!, the Isobaric subs should produce more output than a single-driver sealed box from 200 Hz down to 25 Hz; below that, the sealed box wins, but the curve is already 18 dB down at that point, so who cares.
Well, that's the plan at the moment...next problem is to design the box and build it.