The Sound Labs are very bit the equal of the best Quads, and are capable of greater dynamic range and wider bandwidth. They are harder to drive, so it makes a huge difference as to how good they are going to sound, depending on amplifier choice.
Quads, OTOH, are easier to drive and smaller, and so are better suited to smaller rooms where the Sound Labs would be impossible. What's nice about them (in particular the 57 and the 63) is that they can be driven with smaller tube amps- the ideal combination.
In recent years Quad has been trying to make their ESLs work with transistors (much like Martin Logan) with unfortunate results (hard to drive with tube amps). Anyone working with ESLs knows that tubes are required reading if you are to get the most out of them. So for the more recent models, a set of ZEROs are handy (mandatory might be a better word) to deal with the lowered impedances of the newer speakers.
Sound Labs work better with tubes too, but they were also flirting with the idea that transistors were the way to go, so there are quite a few of them that were built that are nearly undriveable with any tube amplifier. Updates issued about 4-5 years ago corrected some of this, but they remain a difficult load and and you need a bit of power to really make them shine. Earlier Sound Labs that did not have the torroid 'update' (and it attendant impedance issues; IOW had only EI core transformers in the backplate) were a lot easier to drive, but that was a good 10-15 years ago. IMO, these were the best of the Sound Labs, although very recently they have had a diaphram materials improvement that has helped out a lot, especially in the reliability department, something that is a bit of a bugger for all ESLs.
It is very important to note that the drivability issues I have been outlining here are at the root of the question of which is the better sounding speaker! Both are capable of excellence that engenders fierce loyalty from their owners, but if connected to the wrong amps either one can sound terrible! This happens a lot more than you might think- they are both so demanding of the amps used on them that the reality is that there are few few amps that do them justice. IOW the chances are better that you heard them set up poorly rather than ideally. That leads to a lot of controversy!
I speak from direct experience with many customers over the years (about 80% of our MA-2s built have been built for Sound Lab owners, for example). A good number of our smaller amps have been set up on nearly every model of Quad ever made as well.
