Sound-wise, we had a similar thing recently with our Philco 444, reaching the point when warm, as through someone had 'flicked' an internal tone switch to make it sound quiet, dull, flat & no low frequency response at all. Imagine a pillow being placed over the loudspeaker.
Looking at the set yesterday, the output valve wasn't fully home & the centre of the 'speaker magnet was touching the voice coil former(?) The loudspeakers magnet assembly runs warm in use, I'd imagine the expansion was enough to upset the clearance, coupled with the lumpy/rusty bits too.
Having reseated the output valve, de-rusting the inner metal parts of the 'speaker & re-centering the voice coil, it's never sounded so good
My initial fear was, duff output valve.
They keep you on your toes, that's for sure.
Mark