I'm a bit surprised that no one's yet mentioned Quad. I've never owned Harbeth/Spendor, but at audition I've admired both brands' mid-band a lot, but have always ended up going for more full-range alternatives. Same deal with the (older) Quads I've tested.
And, slightly off thread:
To my ear, part of the mid band appeal of all these speakers MIGHT be due to their octave to octave balance - gently rolled at both ends. I say this because so many speakers with this balance strike me as having remarkable mid-bands. In addition to the older Quads, you could add Lipinski, older Sonus Faber, and many single driver models to this list. This effect is also sometimes evident to me on speakers where only the total energy below the mid band is rolled relative to the mids and extended high end. I'm thinking about older Pro-Acs and Merlins here. You may be picking up on this, too.
Marty