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

OP:

Tube preamps use high quality film caps that usually have a life span longer than ours, but failures do happen and they may be small (few millivolts) to larger.

The input coupling caps on amps may vary depending on the quality of the amp. Lots of Yammies used bi-polar electrolitic. 40 years or so?

But worth getting into some details. The preamp coupling caps are rated for the voltage in the preamp, so 650V caps are not uncommon. Amp caps though may only be 50 V. Should the preamp cap fail and deliver 500V, it will absolutely break through the amp’s coupling cap and go boom. :)

If your preamp caps have just been replaced with high quality films they’ll last you a hundred years if they last you five. :)

The real guru on this subjecte though is going to be the manufacturer, @atmasphere who will correct me at length if I'm wrong.

If you haven't bought a tube preamp yet, you could buy a transformer coupled tube preamp, then you would never have to worry about it. Most capacitor coupled tube preamps are fine with solid state amps. Again just remember to turn your system on in the correct sequence because the tube preamp has to stabilize before the amp is powered up.

I tried calling Krell close to where I am, they're in Connecticut. But when I went to leave a message with tech support I got a nice sounding man who said, "please leave me a message". Then his mailbox promptly told me, "voice mail box full". So, I left a message with sales. 50 bucks says I'll never hear back. Not like the old days, for sure. Wife runs the company now, Dan is off selling, well, you know what. 

@erik_squires  , can you explain what happens that makes that "pop" when a preamp is inadvertantly turned off before the amp?

@immatthewj  - It's usually a little DC.  DC = direct current which can happen from leakage or a preamp which doesn't settle at exactly 0 volts.

Easy and save enough to measure.  Any multimeter will let you know how much DC offset there might be in your preamp.  A few millivolts is fine.

OP:  I should mention that most leakage is in the low V range, so your average input coupling cap will handle it.  You can always measure the DC yourself.