Most solid state amps can accept certain amounts of DC to be passed from the preamp to the power amp via existing filters. It is when too much DC is passed that overloads the filtration capabilities of the power amp that these problems occur.
Tube amps are reportedly more likely to pass differing amounts of DC than solid state amps due to the tube fluctuations.
Krell amplifier manuals (at least mine) states that when using tubed preamps that one should activate their input coupling capacitors by removing the jumpers to protect against this more volatile DC from tube preamps. In speaking directly with Krell on this very topic, I was informed that I should find out if a tubed preamp has output coupling capacitors. If the tubed preamp has these, the DC voltage leakage should be well within the "Krell" power amps filtering capabilities. If the tubed preamp does not have the output coupling capacitors, then the input coupling capacitors should be activated (witin the Krell amps by removing the jumpers around the capacitors) for the proper protection, but at risk of impacting the sonic performance of the amplifier.
Since I have a tubed preamp on its way to me (Calypso) which according to the manufacturer has output coupling capacitors, I will not pull my jumpers from my Krell amp. But I feel compelled to send my preamp in after a couple of years to have the caps replaced (yeah, wouldn't that be great if we could keep any component that long?).