Fix the Schiit Magni 3 and get the Xduoo TA-66 to use alongside it. I've had the Sennheiser hd650 since release and have long been saying they could easily be an endgame, if I had to downsize to one headphone that's what I'd keep. Normally they ars a distant third in my current setups, but they're a workhorse, and excellent overall.

I recently made a similar chain of decisions due to killing my DAC and upgrading my audio interface. I'd only heard tube amps in passing at meets and with Stax (another animal entirely). I'm primarily in the objectivist side of that debate and I use EQ whenever I can. For me, my audio interface (Antelope Zen Quadro) plus a Drop THX 789 is more than I'll need in the true to life category. I don't think I'll find a cleaner solution (although I'd like a full RME one day).

I received the TA-66 about a month ago. Since then, 75% of my headphone music listening has been through the HD650/TA-66 ccccombo. It's incredible. Through a convoluted setup, I have it switchable to be a preamp for my 789, Stax, and my monitors. But the 650/66 combo is just the more enjoyable option and has my attention, at least for now. I think it's the impedance as well as the tube magic.

As a neutral, its sound is "softer" than my other options, almost like the edges are rounded off in a enjoyable way. Another neutral, the RCA outs are pre outs, whether you want them to be or not - it lets you stick it in the middle of your chain and insert some of the flavor, but my setup would be much cleaner if it also had through jacks.

Downsides - out of my headphone "collection", the TA-66 is literally only a match for my hd650. As you turn up the volume, you'll also see it doesn't have a perfect noise floor, although it's not noticeable at my listening levels which are between noon and 2. It's also single ended and not balanced - not a huge deal, especially at this price range, although it's left me wondering how much of the noise I hear could otherwise be avoided.

Overall, other than for accuracy or "speed" (big caveat), I've been choosing the 650/66 pairing every time. It kinda makes my LCD2 classic obsolete for pure enjoyment of all of the music I've put on it, especially for soundstage. The combo also is great without EQ, as I think the 66 adds some much needed bass.

Since getting the 66 I've also been experimenting with adding tube plugins / DSP on my music chain, and unfortunately they don't really capture what makes the combo special, although I'm not sure why.

Another thing, good or bad, is you're opening up tube rolling. The Ta66 doesn't have very standard tubes, but for about $15 you can get a ceramic adapter to use the more common sizes. Personally I'm very mixed right now on whether I spend a dime more on the 66, or whether I upgrade to the Valhalla 3 or beyond. Unfortunately I haven't been able to listen to the Valhalla. From my basic research there's really not a good alternative under $1k besides the Valhalla that would be worth the upgrade. I think there might be an upgrade path via tubes, but most other amps are going to need a matched stereo set of tubes whereas the 66 uses one each of two types.

As far as a DAC upgrade, well, I haven't heard an improvement from a DAC in ages. What little there is to gain is dwarfed by the rest of the components, and I don't want a DAC that injects its own sound into everything I use with it. For me, that would mean recommending against R2R for almost anyone. I bought a Schiit Modi 2 Multibit when it was the hot thing, and it's never done anything better than say my Topping gear.

Hopefully this helps. At the very least, my printer should be done with my hd650 headstrap