mckinneymike, hi. I just stumbled on your thread. I have recently been listening to some speakers including the Spendor D7's, so I think I have something to add here. Based on my recent listening tests, I believe the Spendor D7's are your problem. They do not give the typical UK sound that you have enjoyed in the past. As others have said, an SUT change is unlikely to fix your problem. And the comment above about tube amplification is IMO, largely irrelevant. Difficult as it is, I suggest selling your D7's and finding speakers that match your room and your preferences.
As you say yourself, "there has been a seismic shift towards analytical sound in the effort to chase details...". I have directly compared, back to back in the same system, Spendor S8e's against Spendor D7's. These 2 models are less than 10 years apart but the difference in the sonic signature between them is large. The D7's are significantly brighter, more tipped up and less relaxing. IMO, many of the D7 reviews are off the mark and fail to draw adequate attention to the unforgiving nature of the tweeter. The D7 tweeter is too obvious, too strong and too unforgiving of poor recordings for my taste. I preferred the older Spendor S8e's.
Although not cheap, I also compared the Devore Super Nines to the Spendor D7's in the same evaluation, and much preferred the balance of the Super Nines - which was insightful, detailed but too bright. Another British option that might work with your room is the latest Proac D30RS. With speakers, it's definitely best to stay disciplined and listen before you buy -preferably at home - if you can. Good luck.