The problem is that 211 amps are somewhat rare compared to say 845 amps. So info is not going to be as plentiful. Your $350 budget leaves you looking at the usual suspects....entry level Shuguang, Psvane, Linlai, etc. Although my experience is with the 845 and not the 211 tube, I would recommend spending a bit more on the Linlai Global 211-DG. This solely based on my experience with the Linlai 845-DG. So take that for what it's worth. Prices at Premium Vacuum Tubes start at $545 for a pair.