Just to say I am always happy to take advice - I've dug out some old granite? tiles from a kitchen rebuild, put these on the foam 'padding', and put the spikes back into the 753s in the 'parkay' room. I've rerun MCACC on the Pioneer, set the fronts to 'small', subwoofer crossover to 80Hz, and running in 'Direct' mode (so that's 753s plus subwoofer only) even Louis Armstrong's natural sibilance sounds good, as does his trumpet... more importantly the double bass sounds like... a double bass, thanks almost totally to the subwoofer - if you switch to 'Pure Direct' (no SW) then the double-bass player almost leaves the room.
Using manual MCACC, I also rolled off the HF slightly as suggested by Pioneer, as the room is basically hard surfaces only. No curtains and one rug.
I think putting the spikes back into the floorstanders has helped slightly, but it's very difficult to compare back to back.
Yes or no to my solution?
(I can't believe there's a ding in the speaker base!)