No need for a new amp if he gets mini dsp. Amp as is is fineIf you want simplicity, I'd go with the NAD C399 based on what you've said. It's got plenty of power, little to no noise, and Dirac.
Butttttt, if you want something arguably better and for a lot less money, then I'd get a minidsp (gives you PEQ and Dirac), as well as a Topping LA90 for the cleanest, no-noise amp you could ever wish for (surprisingly, it's class A/B, not D!)
....Then I'd spend all that money I saved on room treatment, or a 2nd sub ;-)