I have owned a Pass x250 and a Krell 402e; which I suppose is fairly close to the comparison you mention. I can say after owning the Pass x250; which the 350.5 is supposed to be a much smoother amp than the 250, is an amp I thoroughly enjoyed. I think I may have had a preamp mismatch (Audio Research LS15) at the time; so it sounded a bit lean. However the frequency extremes were outstanding; that is to say the bass and treble were great. The midrange on some recordings; although fleshed out, simply did not have bloom.
On the other hand, the Krell 402e did not have the slam the Pass amp had; the midrange was a tad more liquid, but still lacking something. Perhaps I expected a little more neutral presentation, but both amps for me erred on the side of cool sounding. You can't go wrong with either of your choices, but system matching with SS it a little more critical than with valves; which has been my experience. I would opt for the Pass x350.5; using a tube preamp and Cardas type cables. HTH
On the other hand, the Krell 402e did not have the slam the Pass amp had; the midrange was a tad more liquid, but still lacking something. Perhaps I expected a little more neutral presentation, but both amps for me erred on the side of cool sounding. You can't go wrong with either of your choices, but system matching with SS it a little more critical than with valves; which has been my experience. I would opt for the Pass x350.5; using a tube preamp and Cardas type cables. HTH