thinking of switching from conrad johnson......

Also... the P350 has only single ended (not balanced) inputs while the Pass is balanced I believe so you'll want to consider cables and matching the output of the preamp when you make your second purchase of a pre.

I own a Premier 350 and find it anything but old sounding.  The amp has been a number of reviewers "reference" piece.  They need to change to have a current model item for appearances but tend to hold on to the P350.  It's my backup amp for my c-j LP140 monoblock tube amps but I like to bring it in occasionally for it's more dynamic bass and transients.  There's nothing rolled off about it's highs. Seems to me the biggest difference between the amps you're considering is price.  A P350 will be a lot cheaper than the Pass.