Chris,
With the full range Sound Labs you have the same lack of coloration, the immediacy, the impeccable rendition of transients, the wonderful midrange of the Quads. It is all there. Furthermore you can play much louder, have a wider sweet spot and a far better bass rendition. Placement of instruments is dead right, the soundstage huge with the right kind of software. Also you will have to do much less fiddling around to get the proper coherency between bass, middle and highs, compared to the stacked 57s, with say, super-tweeters and added low end woofers. However to make the sound Labs shine, you have to invest more into good amplification. For both speakers I would say, SS is a no no for good sound. The big Sound Labs are not all that easy to drive. They thrive on OTLs, like the 220watt Atmas or -more conventionally - on the VTL Siegfrieds in pentode mode with their huge power reserves. I also used a 60watter on them with good effect, but then you would lose some of the dynamics this speaker is capable of. Another advantage by the way, over the 57: Dynamics wasn't their strong point, although stacking them did bring noticeable improvement also there.
