Generally it's not the power people buy the 6900 for but the autotransformer. Evidently that component reduces the graniness somewhat heard with the 6300/6500. It's still there, just less so. If you must go with the Mc integrated line, it's clearly the best sounding of the three in my listening tests. You'd do yourself a favor by listening to all three and more importantly other brands.
I'm a Klispch fan since the 70s and owned a half dozen models of their various lines. Every new model produced after 1990 was unlistenable not mention the drop in quality of construction (peeling veneer). I've got a pair of Forte's sitting in another room I can't bear to part with. I'm thinking about mating a Cary SLI-80 with them. I used to use them with pure analog in the mid-80s.
In any case I tried many different SS setups with the many models of Klipsh I owned with mostly poor results. They sound great with a turntable or a tube CD but otherwise the horns are pretty painful sounding. The Forte's sounded tolerable at times with Marantz gear oddly enough. Still be careful before you shell out. I speak from my own experiences.
I gave up on using Klipsch with modern digital and have gone with Aerials which do a pretty good job.
regards, David
