Been thinking my Pass Labs X350.5 might match up nice with the reasonably used priced BAT VK50SE which seems to go for around $3k used. I say this pre because I know it is a pretty dynamic and neutral pre for tubes but it's tubes. Think that might be a good match for the money?
I have my 250.8 matched with a Simaudio 740p and the two together are amazing. I've never heard the 350.5 but based on the forum here I've heard a few folks say the .5 is a little more warm/tube-like in the mid-range where the .8 is a little more detailed.
I didn't find the 740p added any forwardness to my 250.8 (with my Magico S1s) but the imaging and depth of the music exploded. I was skeptical to pay a lot on a preamp after driving a Berklee Dac 2 direct into the 250.8, but I found the sound a bit analytical and a little flat with the Dac direct. The 740p ended up giving me the largest overall gain to my system of any component I've added lately; I was amazed a good preamp could make that much difference. The S1s make any up-stream change very noticeable, for good or for bad.
I would imagine the already lovely midrange on the 350.5 would only sound better with a VERY neutral SS preamp like the 740p or something similar. I've never heard the S5s but assuming they are no more forward or analytical than the same generation S1 I have, I think 350.5+740p or something similar (Pass XP-20 maybe) + S5 in a large room like yours would be amazing.... Cheers