tube preamp w/ solid state amp

none at all. I've been doing this for some time. The only guideline I follow is whether I'm satisfied with the sound. There are rules of thumb for impedance matching (I don't recall what they are, but you can probably find them with a search).

There is nothing inherently wrong with running tube preamp/linestage into solid state or vice versa. Some tube linestages have a high output impedance and should be matched to a linestage with a high input impedance. That is typically the case with a tube amp, but not necessarily the case with a solid state amp. A mismatch may result in frequency response being less than totally flat.

The other consideration is a practical one. Solid state gear takes a VERY long time to warm up, but draws relatively little current at idle, so a lot of people leave solid state amps on all the time. A tube linestage should be turned off when not in use. That may create problems because one should NOT turn off an on components in front of an active amp (could result in very loud noise/damage to the speakers). One would need to find a way to mute the output of the linestage or input of the amp if the linestage is to be turned off and on while the amp remains running all of the time.

Krell power amps require a modification to run safely with a tube pre. I believe a resistor of some sort is put in the circuit to avoid some nasty situations, oscillation I believe, but don't quote me on that last detail!

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 don’t 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.