Hi Richard, as many already pointed out, exceeding the performance of the amp you have without "amping up" the investment very significantly might be a tall order. As you enjoy the sound of M625 S2 but you would like more power, I warmly recommend the Rowland M725 S2 class A/B monos or the Rowland M825 stereo, or the M925 monos. Outside the Rowland World, I might look at Solution amps.... Quite expensive, but yield a musicality that also meets my sound concept.
But there is something you can do to enhance the music of your system while keeping M625 S2, Corus, and the Aeris DAC.... Last week I have installed the recently released Rowland Power Storage Unit (PSU). As I mentioned via email, this is an external full chassis power supply based on ultra-capacitors. It is designed to feed simultaneously Aeris and Corus with pure DC, and takes them off the AC mains grid. I had heard glowing third party reports on it, but I was a little skeptical... While I have not had ultra-capacitors in my system, I understood the concept, and have had battery powered pre-amplifcation before.... And the performance difference between batteries and AC was very subtle.
But when I connected PSU into the Rowland Aeris DAC I realized that batteries supplies and PSU are totally different animals... The performance enhancement is simply staggering, even on my system which I already considered wonderful, where Aeris feeds directly a pair of Rowland M925 4-chassis monoblocks into Vienna Acoustics Die Muzik. The enhancement is apparent from first activation for all audible parameters I can think of, and is followed by a rapid evolution lasting perhaps 200 hours, which pushes performance even futher.
I will soon create a dedicated thread on PSU with a lot more detail.
Guido