Hi Mark,
It's good to see you have a reply from Johndon as you should find everything you need to read from a thread he started!
http://www.avforums.com/forums/arcam-owners-forum/586819-avr350-v-a90-stereo.html
You'll find my thoughts on this matter as well. To summarise my thoughts...I was in the market to upgrade from my Arcam Delta system. Basically, did I need to keep my Yamaha DSP E 800 processor and buy an A90 plus P90 or just go for a 350?
I chose the A80/P90 set-up and am still really pleased all these months on. Purely to my ears, the A80 was better as a pre-amp to the A90 (there are other thoughts on this within the Arcam section). However, when using the A90/P90 against the AV350, the combo won when listening to any 2 channel feed (ie CDs).
I have started a couple of other threads inc asking Arcam fans here if they use a Yamaha DSP E800 and what there thoughts are on it. The E800 is still a fantastic processor that needs a 2 channel separate amp to work fully.
If you have a really good processor (that can power all channels needed ie 5.1), then an A90 plus getting a P90 should be better than an AV350 (check other threads). However, you will have 3 boxes instead of the all in one AV350. Plus, if listening to 2 channel music is not the priority, the Av350 is worth a major shout.
Just to confuse/help even more, the sound when using an A80 or A90 together with a P90 really does improve bass and stereo imasgery etc.... Could be worth finding a P90 going cheaply as it will improve your A90 no end. I wrote a thread on this subject based on all my demoing. May I suggest (you most probably already are thinking of doing this) taking your A90 to an arcam dealer and compare it hooked up to a P90? Then compare the sound to an AV350 in 2 channel mode. You'll be impressed!
Martin