Yes, I did this for a while. If you have the newer OTL version of the MG Head, the transformer output can be directly connected to a power amp. The sound is very romantic, and the bass isn't too bad, though the highs are a bit rolled off (this may be good if you have harsh speakers). The OTL output needs to have an attenuator wired in; if you hook it up directly to your power amp you'll risk frying it. I wired in a 100 kOhm stereo pot and adjusted it to my liking. The OTL output used as a preamp is a lot faster and brighter than the transformer output.
That said, you probably won't be satisfied with either setup. I wasn't. It looks kind of silly and you can get better sound with a dedicated preamp, or even none at all (i.e. a passive preamp). If you're going to go to the trouble of wiring up a wire with an attenuator to use with the OTL output, you might as well put it in a box and use it as a passive preamp.