Subscribed. I can't believe I am just now discovering this discussion. Very nice thread and comparisons, Loquah!
I am glad to see your comparison of the Crack vs SEX. A couple years ago, I had a Crack w/SB order placed when DocB announced the new SEX2.1. I called them up and changed my order (Crack-->SEX2.1), but always wondered whether I made the right decision. At the time, I had not yet bought my HD650, and I wanted to leave open the possibility of lower impedance cans. Also, I like the idea of being able to drive speakers in a close/desktop setup. I have to agree that the soundstage is phenomenal, and with the bass-heavy 650, I don't notice the lean quality that you have noted. I have the C4S assembled, but not installed but I will report back my comparisons after I have time to get them in and listen for a while.
At the time I built my SEX, Doc's output impedance switch was not available. I came up with a simple solution using DPDT switches from the shack. It only offers the ability to switch between 8 and 32 ohm settings. It does not allow for other impedances, nor does it allow for balanced output, but it only cost about $6. I mounted the switches on L brackets which mounted to tube-side mounting screws for the OTs and chokes. I have a few photos in my gallery (http://www.head-fi.org/g/a/693857/default/) to offer a visual explanation. This may be an option for folks who want to have some versatility while they save up for Doc's boards, or those like me who only need it for 8 ohm speakers and higher impedance headphones.
I just recently built the Quickie and have not tried to put OTs on it. But, as a preamp, it is crazy how nice that thing sounds, especially for the small cost. It is breathy but not hissy, solid, and QUIET. It stands up well to my Foreplay 2 with C4S that I have had in my system for almost 15 years. After I get the PJCCS in it, it may replace the old FP2.
Thanks again, for the thread,