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?
ART and Rolls both make decent little mini mixers which you can find at MF or any major retailer. Just be sure to get an active mixer, not a passive one.