Krell 350 MCX Coupling Cap Jumpers

Why not try with and without the jumpers? Ensure your tube preamp is warmed up (10 mins) first. If you’re concerned with DC on the tube pre output, you could test with a DMM on the millivolt range. However, as it’s likely the tube pre has output coupling caps, there shouldn’t be an issue. If still concerned, use cheapo test speakers to ensure the Krell is happy. Then use your main speakers to do the listening test. 

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.