I'm in the same boat as you, owning 2905's and loving their clarity, delicacy, speed and timbrel purity - yet missing the visceral physical impact put out by dynamic speakers during orchestral and operatic peaks. Like you, I'm considering a sub, knowing that believable integration with Quads may be impossible.
The REL Gibraltor series is a candidate, as their published specs look great in terms of speed. Problem is, they are designed to let the main speakers run full-range, while integration apparently is achieved only through tweaking the sub's volume and low-pass level. The connection is via Speak-on units.
This arrangement seems to me a big trade-off: one the one hand, by omitting a Xover, the sub's presence in the system will not degrade the signal received by the Quads; on the other, because there is no Xover to attenuate the high-pass signal to the Quads, their panels will not be relieved of any mid-to-low bass duties that, in theory, might be restricting their overall dynamics.
However, I'm not convinced such "relief" will allow the Quad panels to play louder, because what restricts their ability to do so is simply the limitation of their membranes' excursions before smacking into the stators - the source of distortion, arcing, and shutdown. Which, by the way, I've not witnessed.
There are two expensive solutions to our dilemma: (1) set up a separate listening room with big Rockport, TAD, Evolution or Magico speakers for rock and opera, or (2) add another set of Quads to move more air, thus fulfilling their namesake's suggestion. OK, back to reality:
Larry Greenhill reviewed a JL Audio Fathom f212, mating it to Quad ESl 989's through a Bryston 10B SUB Xover, using a complex setup to calibrate the sub:
Keep us posted on your experiments.