New power tubes lacking low end and slam

@paulgardner-    

        I mentioned rechecking those BIAS switch settings, because: there's the outside chance you may have been adjusting the bias of more than one tube at a time, which would severely affect the dynamics and lows, of the amp.

     If the original tubes sound better, when swapped back in and properly biased*; the new ones just need some time to catch up and: it's nothing upstream (ie: signal tubes).

      Brent Jessee mentions 50 hours or so, of listening/burning, as a norm.

      That's been my experience, as well.

       If the originals also sound weak: try something new, upstream.

                *You DID recheck all those bias readings and switch settings, right?

                       ps: The middle 12AU7 is considered the, "Preamp Tube".