I can't imagine getting speakers that are a serious upgrade on the 3.6Rs. Watt 5s? 30.7s? Chronosonics? (those will get likely you divorced, maybe that's a plus. . .) Nothing else really comes to mind.
The idea of getting some powered subs an crossing over at like maybe 80 hz, that's a pretty decent idea.
The 3.6Rs are a rather difficult load, it's possible your Mac isn't handling the high current demands very well. But I'm not a big fan of this idea that different high quality amps sound completely different, anymore than I think $10,000 speaker cables sound better than OFC 12 gauge.
Although my fave upgrade is the $3,000 power cable - you have like 100 foot of junky 12 gauge romex between the wall socket and the power company, but that last three feet to the amp, that's where the power signal gets wrecked. Yup. Actually a really close second is the $8,000 digital cable, 'cause those 1s and 0s, it's not enough that they get through, they also have to be happy 1s and 0s. Maybe I should invent a split digital cable, so the 1s can flow through blue wire, what with the 1s are obviously male, and the 0s, the female bits, can flow through pink wire. And the pink wire would say "PINK PINK PINK" along the insulator.