Biasing the power stage might be as simple as adjusting a trimpot, or as tricky as swapping out some resistors. Either way, the voltages present on a tube circuit (even after the amp is turned off and unplugged!) can be deadly. Leave that to someone with the right equipment, and the necessary skills.

If the amp still sounds bagged with the the old tubes back in, that means that something else has changed.

Let us know how it goes.