Some developments over the last month. After consulting with MartinC and talking to some dealers, I decided the right way to go for my varied I/O requirements would be a Minisdsp SHD, even though it may duplicate the Graham Slee Majestic pre/dac function. While waiting for a suitable unit to come up for sale (used) I continued room / speaker measurements. A final placing with the main Harbeth P3s at 1/3 positions down the length of the room and dual REL T5x at 1/4 positions on the front wall now gives very respectable response, removing the worst of the room effects.
I managed to snag an SHD recently posted on WAM. Transfering the software licence only took a couple of days before I could get started setting up. There is a definite learning curve to get the software and device configuration right before starting any measurements.
I decided to initially feed a fixed analogue output from the Majestic to the SHD to see how it worked as effectively a 'post processor' taking balanced out from the SHD to GS Proprius monoblocks and phono out to the RELs. Using the basic Minidsp instruction to set up the routing, levels and cross over then applying Dirac gave the first pass at a smoothed response up to 200Hz. It worked but wasn't particulary impressive, hard to describe but sounded to me like a badly compressed cd.
For the second try I removed the Majestic and fead the SHD direct from Lindemann Bridge and used the white paper published by Deer Creek Audio to configure the set up. The result was much much better, integration of the subs worked seemlessly with Dirac smoothing the response up to 200Hz leading to +/-5dB including a room dip at about 160Hz. Very impressive! I've kept this as a baseline to look for potential improvements over time, to try small adjustments to PEQ and XO filters over time.
I found a way to split the output from my Accession phono amp to feed both SHD and Majestic and enable the option to drive the main speakers (no subs) from either preamps. In this case I prefer the sound through the Majestic, to my ears the SHD analogue is, if anything, too analytical and I prefer the smoother Graham Slee sound. So the Majestic stays put.
As for the remaining SHD functions -
-I have not been able to get Volumio up and working but this not an issue as I like the Lindemann interface and it works extremely well with Qobus.
-I have configured one preset for headphones configured so I can take analogue output from the SHD to my ULDE head amp and simply turn the volume of the main speakers. Adding PEQ setting published for the headphones proved very simple and effective.
-The only aspect, so far, that doesn't work very satisfactorily is trying to use the USB output from the SHD. It seems difficult get the output level satisfactory for any 'ripping'. However, the Accession phono amp gives a variable output which I previously feed through a Focusrite ADC to store vinyl rips and will stick to this.
Overall a very satisfactory result, with plenty of options to try 'tweeking' the response over the months to come and I can concentrate of the music rather than fiddling with sub integration.