Although they are derivitives of the same design, they use different power transistors. I don't remember the exact details.
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I think when people say X250.5 sounds better than X350 they are only talking about timbral qualities. I am pretty sure that the X350 would still drive the more difficult speakers better than the X250.5.
I do agree though that a more proper comparison would be X250 to X250.5 or X350 to X350.5
I haven't had the pleasure to compare the older to the newer model, but I do like my X250.5 a lot. I just regret I didn't buy Pass amps before. Really good stuff.
Anyway, from what I've read(I think may be on TAS website), the newer x.5 series sounds a bit more natural, but again, I only read it. So don't take my word for it. I think absolute sound has a comparison between the x350 and x350.5 in one of the reviews......you may want to check their website or search the net for X350.5 reviews.