You shouldn't need to use the coupling caps on the Krell, as long as you turn the preamp on first and let the filament warm up for about 30 seconds. I used my krell ksa 300s with a tube preamp for many years without any problems.
Krell 350 MCX Coupling Cap Jumpers
using a tube pre amp to run my 350 MCX amps
Removed the jumpers in the amp that inserts a coupling Cap on the input of them amp
Wondering to what degree does that hinder performance.