Larryi makes a good point regarding the order of turning things on. If the main amp(s) are on already, or even switched on at the same time as the pre-amp, you could easily get a lot of nasty noise as the tube pre powers up. It is not something you want to hear from your nice speakers. The simple solution is to turn your amp on a few moments after turning the pre on.
Also, I am no expert on the matter, but Gill Audio did change some parts in my tube preamp (a Gill Audio Alana) specifically because I use SS amps - in my case Theta Enterprise monos. As Larryi also mentions, this may have to do with the impedance matching, I really dont know. When I spoke to Gill about my preamp, they suggested these changes to what was already in there based upon my amp. The resulting improvement to what I thought was already a stellar preamp was very noticeable.
FWIW, they also added a time delayed 12v trigger. When I switch on my preamp, it will activate the 12v trigger about 20 seconds later so that the pre will be fully powered up by the time the main amps are triggered on. One switch does it all.
I regards to Pbb's comment, I believe the Krell modification is because it is direct coupled from the factory. This modification is user serviceable and is meant to protect the amp from DC leakage that can happen with a tube preamp
at least this is how I understood it when I had a Krell amp before my current amps. I never did this and never had a problem, but better safe than sorry if you have a Krell, do some investigation first.