Since the Yaqin MC-10T uses 12AT7, I've taken to a bit of tube-rolling. Since this is a relatively budget amplifier, I didn't want to go whole-hog and buy something hideously expensive like Brimars or Mullards. Instead, I went the American route. These are still priced reasonably (for now - so stock up!) My findings so far:
The "flat plate" RCA blackplates - looks like a miniature 12AX7 plate; not folded over wing-style like all the rest, but instead two flat pieces welded together. Very organic and even-handed with a big soundstage. Fantastic!
Blackplate Tung-Sol 12AT7 - Although Tungsol isn't noted for their small signal tubes, they got this one right. Much like the RCA, this has loads of detail but remains cohesive throughout the range of music. It's a close contest here between this and the RCA.
Regular RCA black or grey "folded wing plate"-- very good and very common. Much better than any modern production version, and if you're on a budget, the obvious choice.
Tung-Sol grey plate - 1950/1960s production. Quite common. Though lacking the sheer musicality of the best, it is still a much better tube than the stock Shuguangs.
Other alternatives to try out: G.E. (shudder), early Sylvanias, and CBS. Many other brands will have been made by other manufacturers - for example, Westinghouse was normally sourced from RCA stock.