Steve, have you tried Amarra 2.4 yet?
For the OP, I'd look at getting a Mach2Mini with the battery PSU option. I'm not sure what the guys at M2M do exactly but even with similar specs, I found my friend's AC powered M2M sound better than mine (factory MacMini 2010 with 8GB RAM and 120GB SSD and software tweaks to disable background processes and unneeded peripherals)
For the OP, I'd look at getting a Mach2Mini with the battery PSU option. I'm not sure what the guys at M2M do exactly but even with similar specs, I found my friend's AC powered M2M sound better than mine (factory MacMini 2010 with 8GB RAM and 120GB SSD and software tweaks to disable background processes and unneeded peripherals)