I actually did think of the extra wall wart, but drifted away from the idea. Given your favorable experience with porting the M&M stack around the house, I may reconsider. Having done all the research, something appeals to me more about the M&M than the DF on some existential level. Maybe its the marketing, or just a more attractive set of kit.
I don't have a fixed listening station, but there are really only two rooms in which I'd do any serious listening, three at most. I'd be tethered to the laptop anyway, subsequently tethered to wall power for listening sessions that exceed the laptop battery power.
I can also live without portability outside the home. I have the HD 239 for truly mobile listening and don't require a DAC/amp upgrade for that.
Sounds like the Magni power supply is somewhat custom, and that Schiit's manufacturing and distribution operation is quite lean and potentially ill-suited for special requests. But Schiit still has that small-and-nimble company vibe about it, so hopefully it can sell an extra wart with purchase, or recommend a suitable aftermarket alternative.
I think you'll be happy pairing the HD650 with the M&M. The stack can easily be picked up and moved with one hand--and besides sounding great--it looks really nice on a desk or table. The folks at Schiit seem pretty reasonable, so it shouldn't be impossible to get a spare power supply--have to email them and see, though. No doubt the Chinese factory supplying them could make 10,000 in a week.