IMHO on the subject of SQ in relation to a head unit, if you're serious about it you'll want something that gives you all the tune-ability you need in one unit to make the most of your setup possible without requiring an outboard processor. Genuine SQ units tend to be devoid of the 'flash and pizazz' features designed to attract the Best Buy crowd. They won't have Bluetooth/DVD/SATNAV/SIRIUS/HD Radio/backup cam/custom screen saver all in one, and the best ones are actually single DIN. If you just want to design a CD player with superier sound processing capabilities, single din is enough space to package everything you need to make it great. If you want to design as much fluff as possible into a head unit, double din is the preferred starting point.
I find even at the 1000 dollar pricepoint in the double dins you're mostly paying for convenience and pizazz features, not SQ. At the level and pricepoint that most people are shopping, if you already have a good system to match it with, there won't be enough of a difference that it's worth concentrating on things like voltage and signal to noise ratio. It's more worth your while to get something with some decent tuning features that has the convenience features you're looking for, such as NAV or BT, then go with high quality components and install for the rest of the system. Out of the higher end end double dins, I like the Pioneers for the best compromise between offering mainstream pizazz with some decent adjustability.
This would include the 3300BT, 4300DVD, and the 8000BT for those that can do without NAV and touch screen. All offer 3 sets of 4 volt pre-outs, decent equalizers, and the stuff everybody wants now such as bluetooth and IPOD control. For pure SQ, none of them will surpass the head unit giving up. I don't know of anything in the double-din category that will match something like a 9887 for SQ.