Sorry guys for the late response- after several months I gave up for a reply! I am still very pleased with the 250. There is still a small amount of background hum that was not present with the Sumo, but one can't detect it even on quiet passages of classical pieces, so I ignore it. I still think that it is some sort of ground loop issue, not worth pursuing, and it's not the amp's fault.
One really couldn't ask for more in a SS amp- no distortion, no coloration of the sound, excellent sound stage. You can crank up the preamp to full bore (without a signal of course) and not hear any background noise except as I mentioned the barely audible ground loop. Since it's not amplified by turning up the preamp, that means that it is after that stage. This amp barely gets warm, so it not only has good heat sinks, it's just loafing along.
McCormack himself designed the 225 but not the 250. The folks at CJ are no slouches, of course, so I think that they just made a few minor modifications to the 225 design to "upgrade" it. BTW, the McCormackaudio.com offers upgrades on DNA1 and DNA0.5
Here's an interview with Steve from last year: http://www.hifizine.com/2011/06/interview-with-steve-mccormack-of-smc-audio/
His website smcaudio.com lists his ultimate design for a preamp, the VRE-1 Since it's $17K it's out of my affordability. There is also an interesting product called the Interocitor, which apparently is for grounding issues and can use either balanced or unbalanced inputs and/or outputs. Might be interesting for me to see if I can eliminate that barely audible hum in my system, but for $1800 I think that I'll wait for a review. Or maybe I'll drop him an email and see what he thinks. He doesn't live far from me.
Of course, he also offers many upgrade options on all the equipment that he originally designed, up to the DNA 225 but not the 250. I have no idea if some, all, of none of the modifications are audible, though I'm sure that on instrument testing they are all improvements in performance. I'd have to ask my son, after he gets his electrical engineering degree for advice, but it's a moot point as something tells me that Steve wouldn't touch something that he didn't design.