Rather than hack your power amps (which will destroy their resale value), time to start shopping around for different power amps. Your speakers are pretty efficient, so 15 to 20 watts should be plenty. There are some push-pull 300B amps out there (Don Sachs and I make some, but out of your price range) that are very good. There are some pretty decent SET 300B amps in your price range as well, but they are quite variable in sound, depending on design details like driver design, power supply isolation, and more subtle details like point-to-point wiring vs circuit boards.
I should mention designers of Class D amps sometimes (not always) use an analog front end to deliberately "tune" the sound of the Class D power section. As a result, there is no one Class D sound ... each Class D brand needs to be auditioned to determine their house sound, which you may or may not like.
So ... you can choose PP 300B, SET 300B, or a "tuned" Class D amp. I am a little hesitant to recommend any push-pull pentode amp, because it’s very difficult to get the distinctive 300B sound with that topology and tube choice. There are definite limits to what you can do with cap swaps and power supply tuning.
P.S. Single-ended pentode amps live in a world of their own, quite separate from PP 300B, SET 300B, or PP pentode.