New power tubes lacking low end and slam

I think you have an integrated? If so, you probably have driver tubes for the amp section and signal tubes for the preamp. This should be addressed in the manual.

     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".

In most tube amplifiers, a significant fraction of the gain factor is supplied by the input stage.  You could describe the input stage of a typical push-pull tube amplifier as consisting of two sections, the amplification stage, which adds voltage gain to the signal right at the input, and the "driver" stage, which is there to split the phase of the signal and also to convert voltage to current so as to drive the grids of the output tubes.  Then the transformers once again convert voltage to current to drive the speakers.  Many people use the term "driver stage" to include everything that comes before the output tubes.  But I agree with someone else who mentioned that your input stage tube or tubes, the section that provides the initial gain of the amplifier, may be on the wane.  On the other hand, the output tubes may just need break-in, whatever that is.

OP, how old are your small signal tubes...signal and driver? As lewm adroitly points out above ( better than i could) these could be the issue you are experiencing, along with break-in ( although this I doubt if the bottom end response is severely depleted, as you describe)

I prefer to replace tubes in sets power and pre.