I own a Pass XP-20 and it sounds great with my Pass amps. It is extremely transparent, neutral and quiet. It is also easy to use and has a newer, more convenient remote. I have also heard great sounding tube preamps, as others suggest, in other systems with other amps. I tried the highly regarded Doshi preamp in my system and it was a very bad match for the Pass amps. SS and tubes can both be great IME, but it depends on system matching and your tastes. What's the rest of the system?
I'd get Pass over Krell to answer your question. Others suggest tubes, but that is not your question.