Well, this shouldn't have made much difference but did.
When I first attached my granite plinths to the teak Posselts, I used a 15mm stack of rubber tap washers as the spacers. One day I was going to replace them with wooden "bobbins" the same height, to replicate the original design as per the black pair... unlikely to make any sonic difference, just being a completist.
That day was last week. Courtesy of a donor oak curtain pole, ruler, saw and drill, eight bobbins were made. I installed the bobbins on Monday evening. Gosh. Noticeably tighter bass. No difference in size of gap. I guess the fact that the cabinet is now properly coupled to, rather than isolated from, the granite plinths must be it.
If I'd known, I would have done this ages ago. Very pleased.