I got confused about GND and GBUF, and since I burned the first cheap amp I wasn't to keen on testing further. So a little research later, I bought a new cheap amp and soldered a couple of resistors here and there - and it just worked!
This is enough volume to keep it interesting for now, but I'll probably have to get a better amp later on.
For those who are interested, this page contains the circuit needed. Simple, eh?
I also got receiving status requests from the MP3 Triggers to work at 19200 baud using NewSoftSerial (now: SoftwareSerial). Couldn't get receiving to work at all at first, but I needed to get the latest beta-driver along with a couple of specific pins on the Arduino Mega2560 to get it running.
The timing got off sometimes causing the response to get lost somewhere in cyberspace, but that was rectified using a stand alone power-supply instead of the USB-only that I was using when testing. Wish I knew that a couple of hours earlier....
Still can't get multiple SoftwareSerials to recieve properly thou...
Advice are appreciated!