Best Amps/preamps for Theil cs1 speakers

I am looking to maximize the sound quality of my Theil cs1 speakers. They are currently powered with an NAD 2100 amp paired with an Adcom GFP 555II preamp. The speakers sound very good with this system, but I am not sure that it is bringing out their full potential. What upgrade if any, would provide the biggest difference in sound quality for these speakers?  I have been considering getting another NAD2100 and then running the pair in bridged mode. Will that make a substantial difference? Does anyone have other ideas?  Unfortunately, my budget is limited to $500 for this upgrade. Thanks for the help.