Uhh, just wondering - how loud do you need your Cornwalls to play? And what do you consider low wattage tube amps? I could understand those 2 watt flea amps, if that's what you mean - but I think you have too much power with the Krell, and believe me I know all about the Krell! Now I have a pair of Cornscala's - similar to your Cornwall's and I drive them with a 25 watt Scott 299C amp which is 1/10 the power of your Krell. Using a sound meter about 10 feet from my speakers (about where I sit) If I set the volume anywhere over 7 (OUT OF 10) I WILL DAMAGE MY HEARING! I usually listen, depending on the level of the recording of a CD at about 3.5 to 5.5 on the volume knob, which is pure heaven! Also I didn't spend $2000 on my amp - got it on ebay and had it redone by Craig at NOS VALVES and altogether I spent well under $700 for a fantastic amp. So, to sum up, I believe you have too much power for your speakers.
Anyone chime in if you think I'm wrong.
PS - my brother Denis who is a bass player for 2U, or U2 - always mix them up - came to listen to my set-up and had to turn the volume to 6 to - as he said - "really hear it", but I had to leave after a few minutes as it was hurting my ears. But he did become a huge Bernard Herrmann fan when I played him a few CD's. LOL!