Have I fallen down the rabbit hole, or achieved clarity . . .

I’d say you’ve absolutely achieved clarity. Congrats on the Ushers — they’re on a different planet from the Bose — just make sure you give them at least a couple feet from the back wall so they can breathe and do their thing (keep in mind they’ll need around 200 hours or more of playing time to break in). Why not just run the balanced outs from the DMP-A8 (fixed output — assuming it has that — so you bypass the A8’s preamp) into the front balanced inputs on the Marantz, ditch the 901s completely, and then you’d have 2-channel and HT in the same system? Maybe I’m missing something here, but that’s one option to ponder.

I could do that, but for two things. First, I’m assuming the DAC in the A-8 is better than the one in the Marantz. True, or no?  Second, from what I can see from reading the manual, I’d lose the ability to bi-amp the Ushers if I connect them and all four Bose to the Marantz, or else I’d have to use the Ushers as the front channel speakers and two Bose as the rest channel in a 5.1 setup. Not sure how mixing those would sound. 

I’ve only read that the A8 gets good feedback as a DAC, but whether it’s better than the DAC in the Marantz will come down personal taste — my uneducated guess is the A8 may be technically better as Eversolo apparently put some real effort into it.  I wouldn’t worry so much about bi amping the Ushers as it probably won’t make that much difference, and I’d definitely use the Ushers as the front channel speakers for both stereo and HT and maybe use a pair of Bose as rear surrounds where their diffuse sound may actually work in your favor.