Vacuum Tube preamp with my KRELL KAV 250a, a no no......WHY?

May be worth putting this into perspective.  Your typical SS pre may have +- 15V rails.  You could put your tongue on that and survive.

A tube preamp, like an old TV or amp, has potentially lethal voltages.

I know zero about the technical issues being discussed in this thread. I can speak only of my own experience. I ran a Krell KAV 250a with an Audible Illusions Modulus 3A preamp for over 20 years….without a single technical problem. And it sounded great. YMMV

@immatthewj   you're too funny. Nothing to get off the pot on. Check my virtual system. It's the preamp that I have that's in question.  It has the best music coming out of my system that I have ever heard. That's why I'm scratching my head.

@lynn_olson  thanks so much for that information. It was very detailed and much appreciated. A big wealth of knowledge that I hope others studied as well.

Ps, recapping updates power caps, rarely signal caps.

Krell does not use coupling caps at the input which can result in problems if DC is leaked in from the preamp (both for the amp and speakers). The upside is better transient response as the signal is not impeded a constant charging and discharging capacitor.

Even though the amp will have some sort of DC protection, it may not be enough to overcome a tube preamp DC spike per lynn_olson’s post above. You should contact that tech again and ask if the amplifier has coupling caps that can be activated in order to use a tube amp.