Very well thought out, Mark! I figured I'd throw some thoughts in here, since I was the prior owner of these D25's.
I think you have hit the salient points about the D25 very well. ProAc has taken a more neutral route as of late, and as such the "D" series has taken on a less warm and more analytical character. This is only in comparison to older ProAc's, though, as I still find the "D" sound to be quite warm and musical... But you do have to match the equipment very carefully.
The D25's do indeed sound much better when driven with an all-tube chain (but I have a few caveats here I'll get to). I intially auditioned the D25's with a McCormack DNA-125 (SS), and then just for a few minutes with a ARC 150.2 (SS), and then with an ARC VS55 and VT100(tubes). Tubes worked so much better I never went back to listening to the D25s through solid state.
I will cut (this part of) a long story short by saying that I bought the VS55 that day, the D25's a few months later, and a SP16 very shortly thereafter. I then let the system settle for a few months, learning it's sound.
At the time, my rig was: SimAudio Moon Eclipse LE or Rega P3 > SP16 > VS55 > D25. MIT MH750 S3 biwire speaker cables, MIT MH330+S2 interconnects (the MITs are a tad on the warm and spacious side).
I would say after this period I felt the D25's treble was sweet although not the last word in extension, the midrange to be communicative and gorgeous, and the bass to be full and overly ripe at one particular frequency. I did notice this ripeness, although I would say less so than you; this was something that only a small amount of recordings in my library exacerbated (say, 5-10%?). In particular I remember Preservation Hall Hot 4 with Duke Dejan to become strangely lumpy in the bass line, so much so that it drove me to distraction. One issue for this was the D25's: they are ripe at certain low frequencies, I think. The other was my room: it exacerbated this. Not much I could do.
Ahh, but I could do some. I started tube-rolling with NOS and new production tubes. That is a story unto itself; but suffice to say I settled on a power tube (TS 6550 reissue, although I flirted with some NOS TS6550s) that provided excellent grip of the lower frequencies, far more extended highs, and better soundstaging. I rolled preamp tubes based upon my mood and whims (mostly Mullard 50's shortplates, although I did use some Telefunken 12AX7s when I wanted more neutrality). This provided more grip and control, while injecting an absolutely gorgeous midrange that would pull me into late, late listening. During the tube rolling exercise, the ProAc's revealed all upstream tube changes immediately. This was my first "real" tube rolling (back to back listening of MANY different tubes) and it really sold me on the value of rolling. The ProAc's were simply windows into what was going on upstream.
I wouldn't characterize them as completely neutral windows; they were editorializing a bit... Warming things up... Sweetening the highs and communicating the midrange above all else. This was VERY musical when combined with really good upstream tubes. At no time did I find the ProAc's clinical, sterile, or would I have ever characterized them as not being typically ProAc midrange champs. They just pulled me into the music in a way no other speaker yet has. They are a bit on the "wet" side of things, so if you like the B&W analytical sound you won't like these. I never did completely quell the lumpiness in the bass, but honestly it wasn't a factor in the end. I loved every minute the D25's were in my house; after settling on a good upstream mix, they were nothing short of the most musical speakers I've ever heard.
In your case, I think one test is definitely in order. You need to try some new pre tubes. The 12AX7EH's you have in there are, I would characterize, a bit round and ripe in the bass, with a slightly recessed but golden midrange and highs that go on forever and get a bit spitty up top. I'm frankly quite surprised you weren't drilled in the head by the highs--but such is system synergy, and the 100.2 must be a little warm in that arena. For midrange magic, I'd try some Mullard's, both the longplate and shortplate if you can get your hands on some. I just don't think any new-production tube is going to do it for you in this case. The midrange magic of this tube will make you completely forget the Harbeth's (well, maybe not, but it will sound gorgeous!) Some reccomend the new 12AX7M GT reissues, but I don't think they come close to the magic of the originals--the tone is similar, but the magic is gone.
But that won't tighten the bass much. Just a tad. And since you don't have the option of running new power tubes, maybe some other trials are in order here. The new-production Ei 12AX7 would be excellent in that arena, but it won't have the midrange lustyness that you'd like. I think maybe some NOS TungSol 12AX7s. I didn't run those for very long in the SP16, preferring the Mullards overall, but my listening notes convey they had great midrange magic and perhaps a little more control than the Mullard tone-machines. Come to think of it, I think the TungSol's would be the ticket, at least from what I've heard.
I will concede that maybe even this won't make the D25's totally amiable to you--I still think you need a good tube amp to get the best of them. But the pre tubes are certainly worth a shot... You're just not hearing what these D25's can do. And with the right mix upstream, they are very good indeed.
