first, derrick did give you some good advice about the "benefits" of passive bi-amping
second, have you had the M90 re-capped since you bought it, assuming you are original owner? I don't know if I can give you a direct answer but that amp is gorgeous! it makes me nostalgic for older Elite gear...Pioneer sure did make some great stuff.
based on what I was able to google on its specs, it will be a more powerful & somewhat cleaner amp, probably class A/B but perhaps biased into class A for first few watts. its only 200 watts vs the 68's rating 140 which isn't a significant difference but look at what's behind the specs. M90's THD numbers: 0.003% at rated power. the class D amps Pioneer uses do have very low THD levels in the middle of their power curve, but as they approach their max power before clipping, the distortion on high freq's quickly rises. that is the nature of class D amp design. look at Pioneer's rated power for the class D amps: 140 watts but that's at 1kHz (not full 20-20Khz rating like your M90) and all channels rating is at 1Khz and 1% (clipping). the class D amps are awesome amps for home theater receivers, able to maintain their power for 5&7 channels but IMHO they are not going to beat (or match) your M90 for the cleanest power approaching clipping. this all assumes the M90's capacitors are still in great shape.
your decision also depends on how efficient your speakers are. Klipsch's usually are pretty efficient so usually wouldn't need that much power. so you probably could ditch the M90 & not miss anything. OTOH, if you had inefficient speakers like I do, I know I'd keep the M90. try it both ways. but I know I'd be seriously tempted to keep the M90 in the system just for its vintage appeal & looks let's put it this way. if big majority of your use is movies not music, and eliminating 2 interconnects + power cord and freeing up a shelf space is important to you then I don't think you'd miss much with Klipsch. but the M90 will be the theoretically superior amp when pushed to high power peaks, if it doesn't need re-capped. whether that is audible or not, I can't say. but it sure would be nice to look at in a rack
if you do decide to sell it, you might want to put it on AVSForum classified or Audiogon or let us know
reminds me of Pioneer Elite's heydays.
Edited by ss9001 - 7/8/13 at 3:54am