Check around on http://www.head-fi.org/ -- there are several entire threads devoted to tube rolling the MG Head. I'm using NOS Sylvania black-plate EL84s and a NOS Telefunken 12AX7 -- paid about $70 for all three on eBay.
Start off with the 12AX7 -- that's the "preamp" tube, and it will probably make the biggest difference in the sound. Lots of people have experimented with about every 12AX7 under the sun, as well as other tubes like the 5751 that also work (I'm not too clear on what all tubes are "equivalent," myself, but the info is all in the threads on head-fi.org).
Start off with the 12AX7 -- that's the "preamp" tube, and it will probably make the biggest difference in the sound. Lots of people have experimented with about every 12AX7 under the sun, as well as other tubes like the 5751 that also work (I'm not too clear on what all tubes are "equivalent," myself, but the info is all in the threads on head-fi.org).