OK. Here we go…
I need to put this brief ‘mini review’ in context. Having young kids, I do quite a bit of headphone listening. The very best headphone sources I have used in the last few years are (in no particular order) the original Chord Hugo, the Chord Poly/Mojo 2 combo, the Sony NW-WM1Z DAP and the FiiO M17. If you asked me to choose ‘just one’ for SQ alone, it would be the Sony. It’s an absolute masterpiece, and sounds fluid and composed.
But there’s a problem with both the Sony and the original Chord Hugo. They’re not Roon-Ready. And that’s a deal-breaker for me. Hence why they’re no longer with me 
Enter the Chord Poly/Mojo 2. This combo sounded great! And it had plenty of power to drive my 600ohm Beyer T1.2’s. But the battery-life (even when it was switched off) was absolute awful. Unless it was charged well-before I wanted to use it, it was DOA. I had to almost pre-empt myself. Am I going to listen to music later? If so, I’d better put the bloody thing on charge because it will probably be dead after being switched ‘off’ for a few days. Annoying. Plus it lacked balanced outputs to use with my balanced headphones. It’s now gone too.
Maybe the FiiO M17 would be the perfect solution. A Roon-Ready end-point and and a portable DAP! What’s not to like? Listen to it in isolation, and there’s nothing to dislike. This monster sounds absolutely sublime. It’s literally a ‘hairs-breath’ in SQ from the Sony, AND it’s Roon-Ready. Plus it has endless power to drive full-sized headphones like my Senny’s/Beyer’s. So it’s perfect? Well, no. Unfortunately, nothing’s perfect, unless you’re sat in a ‘cafe’ in Amsterdam 
The M17 is (very) heavy for a supposedly-portable device, and puts out a lot of heat. Plus, the battery-life (although better than the Chord combo) is limited. At least it keeps its charge when you switch it off, unlike the (useless, IMO) Poly/Mojo 2. But, oh! The SQ, especially with Roon 
Could there be anything ‘better’? Well, maybe the newly-launched desktop R7 would fit-the-bill…?
Spoiler Alert! Sorry folks, but the M17 still takes the gong in the SQ department. The R7, however great, lacks the three-dimensionality of the M17, and also lacks its sublime bass extension. Although the disclaimer is that I’ve got a brand-new R7 out-of-the-box, and I’ve only been listening for a few hours. But it has a lot more power than than the M17. I’m using the R7 with FiiO’s PL50 LPS. And I’m more than happy right now:
But there are a lot of pluses with the R7. It’s absolutely dead-quiet, like the M17. It has a sound-signature identical to the M17, and has literally endless power to drive just what you want it to. Plus, there’s no excess heat, and no battery-life concerns. Couple this with a 4-pin XLR output (obviously lacking on the M17) and I think we have a winner on our hands.
Oh, but I’ve forgotten one important fact! The R7 is half the price of the M17!! Does this change any anything? IMO, it changes everything. Is the R7 better than its stablemate, the M17? No. The M17 is still ‘king of the hill’. But is it a better buy? A resolute ‘No’. The R7 is by far the better buy, and if you’d never heard the M17, you’d have no idea what the f*** I was talking about. If the M17 was a 10/10 in the SQ department, I’d give the R7 a solid 8.5/10. It’s that good. And that’s without considering all the other ‘pluses’ mentioned above.
Put it this way. I’m a few hours ‘in’, and I’m still listening. I have no desire to gravitate back towards my (albeit breathtaking) M17 for ‘desktop’ listening in my bedroom.
This newbie from FiiO is a KEEPER.