Just a note, but a properly done BillD mod slightly alters the gain on BOTH outputs.
A C1 cranked full throttle (BillD or not) will likely drive any amp into distortion.
Something doesn't sound right. I would think that they should at least get loud but would clip and get distorted. Infinity's require good current but Kappa 7s are not amp killers like 9s are. My Infinities get loud but are very clean. Don't seem loud at all until I walk out of the room. Then I notice how loud they are and go turn them down. If anything I'd say its the preamp. I know Dennis adjusted it when I had the mods done. Gain issue?