I currently run an external power amplifier (in fact without a receiver for now) because I come from a recording pro sound background. I have Polk RT-55 and just got a new set of B&W 602 S3. I did the following amp comparisons with a QSC RMX 1450 and a Carvin DCM-1000. I use a Carvin DCM-1000 100% of the time, and got a chance to trade someone for their QSC RMX 1450 if I wanted it, so I took it home to do some listening tests in two different rooms with very different acoustics. Note my results were far before I ever went online to be "influenced" by other people's opinions.
I noticed a slight but noticable difference of the upper mid to high frequencies on the QSC. They were a little more harsh than my Carvin. I only noticed this for the first two seconds after a switch. I would not have noticed a difference if there was any more than 10 seconds in between the switch. I liked the slightly smoother sound of my Carvin. The bass however was a bit different. I believe the bass response of the QSC was more "defined". It was a little clearier and punchier than my Carvin. Noticable? No, but I dont believe to many amps would give a "significant" difference in sound - at least not as much as the sub you use.
I have heard that the Mackie amps are very common in studios for reference monitors. I have heard their own active reference monitor a while back, and remember it sounding very very good.
I think most of the stuff you read on the internet about QSC is most likely there cheapest RMX amps. I would imagine their PLX amps are a bit better. And just remember, you can bridge your amp to run that subwoofer. I can't imagine you will need more power than even the smallest amplifier they make.
Just one other thing to think about, which is something I'm running into now: I have an external power amp, and I am looking to get a receiver primarily to be used as a 5.1 digital decoder as well as powering maybe 2 surround speakers and a center. My issue is that I have about 200W per channel at 8 ohms, and like the gain very very low. With a receiver I will have to turn the gain up very high, because the pre-amp outs will be reference to your receiver's volume. This scares me turning it up that much. If a wire ever got unplugged and any kind of pop or crackle was to occur, I would be very concerned about damaging my little speakers. With your monster sub - well that may be not a concern. But the physical structure of your house may!
