I have a TC Electronics RH450 amp head. It has an aux input, but the signal is only available at the headphones, which is a shame. Sometimes you'd like to hear it through the speakers too. A solution for any amp with an FX loop would be to use a small simple mixer to blend the "preamp out/FX send" signal with the drum machine or mp3 player or whatever, and route the output of the mixer to the "power amp in/FX return" socket. The signal will be mono, of course. Anybody knows of a simple mixer that would do the job?