Power tubes need to be matched for gain (transconductance or gm) as well as for current and both are equally important. Individual biasing does in effect match up tubes for current output, but it does not address gain. For that, you have to match the tubes as well. Also, even with individual biasing, it's better if a pair of tubes are in the same ball park for current ourput as well. My own rule of thumb, with individual biasing, is to match tubes within 3% for gm and 6% for current.