The classic speakers often used by D-clone users are the Celestion g12-65 and the EVM 12l.  I have found something better than those, but out of production...The MI and G series speakers from JBL.  Specifically, in the MI series, the MI-12 and the MI-15, in the G-series, the G125-8 12" and the G135-8 15".  MI series have a 2" flattened ribbon wire VC, rated for 150w RMS, the G-series use a 3" flattened ribbon wire VC rated for 200w rms.  All of these speakers sound great at lower volumes, and simply tremendous at higher volumes.  There is great control over the cone, no wierd sonic artifacts.  Clean or dirty, I have not found the equal to these speakers and I have heard LOTS of speakers.  Easiest to find on ebay.  There are some NOS MI-15's on the bay right now, I got two of them and they are wonderful.