Lyr 3 + Mimby.
Or - and this may be unpopular - the Bifrost 4490. It's actually a pretty solid sounding DAC. You get Gen5 usb, matching chassis size, and better laid out 4490 implementation.** I know most people have it hard-wired in their brains to go multibit, and for the HD800 I would strongly consider it.
With that said, I've never been enamored by the Bimby vs Mimby especially for the price vs performance difference. Having had both at the same time, I'd be damned if I could suggest a $600 DAC vs a $250 one just to get the "next level" multibit or to get a stackable chassis.
I ordered my Lyr 3 with the internal multibit card, and directly compared it to the Mimby. Mimby gives you better sound stage and better input options. You can keep it powered on all the time too, which MB needs to sound uniform. (It has a warm up time before sound stabilizes.) For the extra $50, it's a no brainer.
Also, and don't take this wrong, but it seems like you should do more homework. The Lyr 3 vs Valhalla 2 are completely different sounding amps. You should decide which will provide a better sound synergy with your headphones, not buy one amp or the other based on price and/or size.
Disclaimer - I've heard the Valhalla 2 quite a few times, but have never owned it. Same thing with the HD800.
My money would be on the Lyr 3 given my experience with both of those amps.
Just my 2 cents.
** Edits in italics.
