KT-88 vs. EL34

I wouldn't say better, its more like different. 6550's and KT88 have a more "modern" sound to them. Highs are more extended, and bass has more definition. EL34's tend to focus more on the midrange. Keep in mind that I'm talking in general terms here. There's always exceptions.

If the amp was modified, do you know if its wired in triode? It makes a nice difference. 

"I am currently using Canton Karat M70s, woofers facing out. Not too efficient and 6 ohm average. Current amp is SS, a B&K AVR-307 (150w x 7) in two channel analog mode, used for stereo only."

Since you already have the gear, you might as well try a vertical biamp. Remove the jumpers on the back of the speakers and use 2 channels on the amp for each speaker. 4 channels, not 2. It may make a big difference, or not. Results vary.

I thought you had a seperate multi channel amp, not a receiver.