I try to run all my amps at about 40ma per tube even if I do not measure plate voltage when biasing. Most amps run 400-450v and 40 is nice number. Not running cold for sure but not hot, just warm. I would think its the grade-rating of the tubes and the bias pot can only get them down to 40. I could be wrong but that's what I would guess. Your new tubes are probably on the far end of the spectrum as far as their rating. Just my guess. You could contact the tube seller and explain and ask for a replacement set. See what the amp designer says to as mentioned in previous post and maybe that info will help if need to deal with a replacement set from seller. You could also just run them at 40 and leave it.