On Brian's advice, I purchased a pair of "upgraded" mid-bass drivers for my VMPS Tower IISE's. There were no crossover changes required and the drivers were reasonably priced, so I decided to roll the dice and go for it. They arrived yesterday, but I was surprised to find that the drivers had foam surrounds and a smaller magnet. At this point I grew more skeptical.
OK, so I popped out the old 12" drivers and soldered the new ones in, then cranked some tunes. I was immediately impressed with the improved separation, detail, and soundstage extension. The sound was almost precisely what I had longed for.
However, there was a significant decrease in bass, and that got me a bit worried. After a quick e-mail to Brian, he told me to pinch some putty (if you don't know what I mean, visit the VMPS website for more info). Again, I was skeptical. So I reached underneath each speaker and took a fingernail full of putty from the passive radiator, then played some music. I was absolutely floored by the result. That tiny little bit of putty nearly restored all of the bass that was lost. Of course, I took another fingernail full and got more bass, then another, and stopped at three putty pulls. Now I have improved detail and separation with the bass intact. Gotta luv it, man.
Lesson for the day -- get yourself some tweakable speakers. End of lesson.