Mating Quad 2905's with crossover and subs?

Undougtedley you'll get more pronounced bass if that is what you want. The amps in subs are optimized to funct within a certain frequency gain. I have heard your speakers but only for an audition in a store. It would be very easy to overwhelm the Quad's out put in the in the mids and highs. I found them to be a very polite speaker, youll have to dial in the subs ver carefully not to blow the rest of the music away.
I should have said at the outset that using an external crossover will work in dividing the signal for the subs.

One of the problems with sub's and Quads is the speed issue. Quads are very fast and clean. Dynamic subs, no so much, unless you have small cone drivers properly implemented and even then the frequencies handled by the cross over slopes will lose some clarity. So if you are at all fussy about sound quality, and I assume you are for chosing Quads in the first place, I would not go there. I tried it and never go it close to right.

But all is not lost! First lots of folks put the Quads on stands. This can put a hole in the upper bass and make them sound thin in the upper bass/midrange. The were originally designed to be on the floor! And, with very careful positioning, on the floor you can get relatively flat FR to 35hz. And, once you have heard good electrostatic (or planer) bass you will have a hard time listening to dynamic woofers. :-)

I went back to dynamic speakers some years ago, but not because of any issues I had with the bass part of the FR.

If you must have loud sub 60hz bass, be sure to get a high quality cross over which has a steep slope roll off of the sub's FR above 60 hz and run the Quads full range.

Good luck.

Wilson Benesch Torus sub is the best match I've heard. Unfortunately, rather expensive and hard to find. The Torus has the speed without the big box woofer bloat.

That said, I've always liked Quads without a sub in a medium sized room. I've owned at least five different subs used in conjunction with my box/cone speakers, and in the end, never kept any of them. Perfect integration is almost impossible to my ear. Another idea might be to try some Martin Logans that have the sub already built in.