If you want to try a different pre-amp, then you need only spend a fraction of your budget. If looking at passive, look for a decent transformer based passive rather than cheaper units with a regular potentiometer or resistor based stepped attenuator.
Passive units are ideal for a second hand purchase as there is little to go wrong, hard to damage in transit and tend to depreciate heavily as they are rather too hair shirt for the regular hi-fi market.
As mentioned elsewhere I can highly recommend the Glass house C-core TVC passive with my Net audio Quad 405 power amp, see article below. I bought mine second hand on this site for similar money to a Quad 34 and sonically it is top notch.
I have also heard my 405-3 sound really good on the end of a higher end MFA TVC pre owned by Montesquieu. I also owned the Mk 1 MFA classic which was good but not as full as the Glasshouse.
What features do you actually need in a Pre? Phono ? Multi input switchings ?
One can assemble a 'good as it gets' Passive pre.. Using a stepped attenuator Or a Transformer based attenuator (big difference in costs tho)
And a simple Buffer stage (Passdiy.com B1 kit ~50$) ..to preclude any and ALL impedance matching issues which sometimes lead people to believe that passives don't work.
With the above you likely cannot spend enough to better the sounds (Spare me)
Also it's IMO, Massive overkill.. for the geriatric Quad..regardless of mods.
That will however not provide a Phono, or easy Input switching.
Lotsa V good and affordable Phono stages available.. in DIY/kit form.
Sadly, commercial /hi fi shoppe..ones are Usuriously priced for quality delivered. (much like pre's :-).
Many ! choices.. as always..good luck in your adventure
Thanks again Serge.
From what you say, would I be better changing the caps on my current crossover for new ones (Falcon Acoustics have sets) rather than going for the less used boards currently on sale?
Or maybe leaving well alone!