Recently purchased the HDRX-20. It came with matched Tung-Sol 5881's. They were factory set to 27.5ma bias. The tubes were well matched to 1/2 of one ma. apart. They sounded better to me at 27.5ma although I did try the recommended 29ma which distorted better. I would like to try out a matched pair of Sovtek 5881/6L6WGC tubes. They have the wafer {disc} bases. Do I stick with the recommended 29ma bias? These are Russian 6p3s-e equivalent tubes with a 30watt rating vs the Tung-Sol 23watt. Plus, the Sovtek's are mil-spec., the Tung-Sols are not. I don't expect or need any more watts or stress to the tranny but I'd like a bit more clean headroom and reliability they seem to provide. I do have some spring retainers because the bear traps will rest on the glass {due to the short base} and I'm particularly not a fan of them. Any thoughts/experience will help Thanks, Chuck.