I tried that once. It was still just as loud. The headroom was about the same. I think the tone lost some body and was a little less 3d. After a few hours i went back to 4 tubes. Did i get an instant 2204 from my 2203? Not even close.
I used a Carvin Tube 100 stereo power amp with my all tube Carvin Quad-X preamp as my main gigging rig for well over 10 years. That was a sweet setup. That poweramp was stereo, an input on each channel and each channel was essentially a 50W amp with its own volume control. Each side had its own output transformer in push pull - two power tubes each side.
So I am thinking if his was stereo he would just turn the volume down on one side. Maybe that is why we have not heard back. On my Carvin, if you pulled two tubes it would;d have to be on one side which would essentially just be shutting that side down.