basically i bought a used peavey vtm 60, the power tubes went super silent and so i decided to change them. bought a set of sovtek 6l6 5881 wxt's and they sound super flabby, a little muddy, just generally really, reeeeaaaaally loose compared to whatever came in the amp. is this just the natural characteristic of this tube? or do i need to re-bias according to these tubes?