You could use anything on top maybe tubes.Have done it with my KSA 300S.But as a I was telling my buddy whom I got a Byrston 4B ST for to power and who had the little passive x-over made by Maggie with some mid fi Denon's and MGII's we crapped the next step would either bigger amps or getting another Bryston and a external x-over like the Bryston 10B which would sound better than their larger dual mono 7's (or is it 14 I forget now).
All thing being equal to get the best out of that Krell whose grip on the bass is superb and another amp of your choice for top getting either a Bryston,Marcha tube or for cheap NHT etc makes bi-amping realize it's full potential.If you have two amps made by "x" manufacturer at say 100 wpc channel you only get 150 wpc by bi-amping.So most manufacturers will tell you to use the mono switch and on average you double the power.But you do ghet better driver control with horizontal bi-amping but to get it right you need to adjust a external x-over to get the crossover and slope that fits both the speaker and the amps.
If you want a sweeter top and and less of metallic "etched" mid range get a Marchand x-over and a good tube or sweet SS amp fro up top.I sold Krell and loved the bass and thought highs were clean and detailed but the midrange was what made it as a brand bot a final match for me.Your opinion and millage may vary but I would not go passive if you can swing an active x-over withe what sound you want up top with enough power to be used with Krell.
Cheers
Chazz
