Hmmm, I would attack either your C2 processor or your Theta amplifier.
If you like your C2 processor, maybe Krell Foundation? Or Classe SSP-800? Both are clean sounding processors.
If amp, maybe get a couple Parasound JC1 monoblocks for left and right. Then a Parasound A31 for center/surrounds?
It's a hard call, because I totally understand that your speakers might be a weak link here in your system.