I think the Quad 12L may be the closer sonic match to the Twins with the 6-1/2" midbass. $1000 retail vs $850. The 11L is $700 so the Twins fall right between the 12L and 11L.
I have had the Twin MKII in my home for a demo last summer, they were nice musical monitors. I now own the Quad 12L and prefer its overall sonic nature to the Twins. I haven't heard the MKIII twins but the Quad 12L is the best monitor I have heard on my electronics in my home in all my 20+ years in audio (as if that means anything to you). The Quad 11L is suppose to have a bit smaller soundstage than the 12L but similar superb sonics. The tweeter in the Quads is fantastic, one of the best I have heard. The transition from mids to highs is very smooth without being softened as I found on the Twins. Imaging, bass, mids, highs, all of the above comes in musical spades on the Quads.
Can you tell I love em'?