I hesitate to even interact with you given your history on this site, but the 12 cab he owns does not have a tweeter (that is what the NT means), and some versions of the Powerhouse 15 are one way cabs also. So, we need more information. Also, a tweeter up or down has no impact at all on the performance of the woofer, so that is most likely not valid solution, even if he does have a single tweeter in the Powerhouse.... which we do not know
There is no reason that bass can't sound fantastic through that rig. The tone stack of the TH500 is complex, and many TBers have had a bit of trouble dialing it in at the start, since the manual is pretty sparse.
It is ALWAYS good to make sure the OP is aware of some of the quirks of certain pieces of gear prior to recommending dollars being spent.
The various discussions about favoring the neck pickup on J for more lower midrange output might actually be a helpful suggestion.
That all being said, please don't blow up this thread.