Can't help a lot, but I can't see how its the speakers. Even if you could load your amp in a way that it wouldn't clip, you should still be able to overdrive the amps first gain stage and create distortion with modern separates like these. They tend to have high output and sensitive input so that half to 3/4 on the knob will clip or overdrive on loud passages. My speakers are basically the same sensitivity and 125 w/c will literally unlatch doors and make stuff dance around on the countertops, you will know when your getting 200+ watts.Since your amps have input level pots, hook a cd player direct, and turn them up with the trim pots. I ran a no preamp system for years, and have never seen an amp that a good player can't overdrive. That will atleast tell you if it's the amp or the pre.
Thomas