I second the vote for the Svetlana KT 88 vs 6550 (haven't tried KT 90's). Just retubed my Aronov monoblocks with KT 88's from Upscale Audio and the improvements in the midrange and the top end are dramatic. If the bottom end is less defined, I sure don't miss it. Then again, my monitors only go down to about 45 Hz, and rest is handled by a powered sub.
