[Monolithent]

Could you tell me exactly which model amps you are using? There may be another way to get you connected. Might even be simpler.

Basically, merge your outputs from your mixer into a single XLR connection (you can find adapters for this at most music stores) and run them into one (only) of the two XLR inputs on your amp. The other XLR input is often setup to serve as a line level output when operated in this manner (there is usally some kind of switch on the back of the amp to set this up). Connect an XLR cable from this connection to the input of your other amp and there you go. Both connected and far simpler set up. This is how I run my monitor speakers. The downside here is that now you will have to operate in mono instead of stereo. If you are leary of this then I can tell you from experience that after running mono for over a decade. No one has ever noticed. Plus you can also bridge your amps and run them at 4 Ohms with these types of setups. More power and actually easier on your amps as long as its controlled.

The USB port on the ProFX12/8 is used for streaming the main mix to a recording DAW software and sending a stereo stream from a computer program (such as hoster or a DJ program). I believe its level is controlled by the USB input knob near the EQ sliders. I don't think it is possible to reroute it to a slider of any type except the main output. If you are insterested in one that allows you to connect to each line you might look into a Yamaha N8 or N12. The cost is higher but it allows you the ability to run several items back and forth to your computer simultaneously via single firewire cable and is assignable to input sliders. Stay clear of the M-audio NRV-10. Its crap for live purposes. From what I've seen the USB connections on mixers are all single stereo in and out (even on some of the higher end mixers like the Allen & Heath ZED series which are far better built) which gives the firewire units a strong advantage in that they have multiple stereo ins and outs.

Good luck with the Mackie. I have looked at it and it seems pretty solid and should serve you well for quite some time. The main reason I didn't go with it is that I am moving into a lot more live sound applications and there weren't enough connections plus I prefer firewire. USB has too many issues for my taste. I am waiting for USB 3.0 to see if it has resolved them.