New power tubes lacking low end and slam

When you say same as the prior original, do you mean KT120 same? Or JJ, Tung-Sol, etc same? 

@millercarbon Same Tung-Sol KT120’s

Burn in can be a painful process. I've been known to walk out of the room because I couldn't stand the sound. Just let them burn in. They will change.. if they are the same tube they should sound the same after burn in.

You may want to check your driver tubes. I replaced these recently and the difference in low end slam was considerable. 

@daveyf what are driver tube? I was only aware of power tubes and signal tubes. 

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. 

@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.

Let them bad boys settle in, if you changed the 120 tubes with the same tubes and biased right, all will be well.  Enjoy the music

Thanks everyone! Appreciate the ideas/thoughts.