Have $600 Need input on upgrade for Acurus Amp

I am looking to upgrade my Acurus A-80 amp and would appreciate some input.

Need something to deliver more power to my new 4ohm RS-722s from Parts Express (speaker kit that uses Dayton Reference Series).

I am a big fan of high value / bang for your buck audio and would like to spend no more than $600 (new or used). The A-80 has served me VERY WELL so I am considering a more powerful Acurus amp but would like to explore some options? What about Parasound or NAD or.....? Any thoughts on the Pro Audio offerings? The Behringer Amp looks interesting but seems to have some quality control issues?

My pre-amp is an Acurus RL-11.

Thanks in advance,
Alex