@Mr Ee ?
I think he‘s got the K11.
I do, k11, k11 r2r and am seriously thinking of getting the k13 in a couple of weeks, and although it (k13) would probably be mainly for headphone listening I'm sure it'll have a stint in the main setup too.
If (we all know it'll be when) I get one I'll comment in case anyone cares to hear my thoughts on what is charitably a 13 bit dac
I used the two k11s with a variety of wiims, mainly pro, ultra, amp ultra for both headphones and feeding them into my Hegel H190 I had at the time.
Liked the k11 for headphones. Significant improvement over the (OG) Ultra headphone socket.
Didn't like the k11 as a dac at all. Strange.
k11 r2r, I preferred the OS mode over NOS. Fairly subtly, but noticable, difference. I think that's 'cos I prefer a cleaner, more precise sound. So why even bother with an r2r dac then...?

I do think the k11 r2r was easier to listen to at loud volumes for longer than the H190 dac.
I listen to a lot of rock, metal, pop, dance and appreciated the clarity of the H190 for those, but I specifically liked the k11 r2r for slightly more mellow music, such as Dire Straits. Private Investigation - sounded sublime on the k11 r2r.
A couple of issues with the k11 r2r (and maybe the k11, tbh I can't remember as that's now in another, less frequently used headphone setup).
Using usb I sometimes (often?) got a slight click between tracks, even when the sample rate didn't change.
Using optical, even though specs say 96 max I often had issues with 96khz. And yet a few weeks ago I put the k11 r2r into my main setup for a few days and I'd swear 192 worked over optical. I can't imagine the firmware update I recently applied fixed that, surely?
And to bring this slightly back on track - using an external dac with an amp ultra, which is what the thread is about.
I have an amp ultra usb out to an rme adi 2, and I still haven't got to the bottom of why as soon as I make this connection I get a lot of reasonably loud clicking from the speakers. So for now the rme is fed optical from a pro. Good job I have too much Wiim stuff! (Is too much even possible?

Seems to depend whom in the house you ask.)
In the next few weeks I'll be completely dismantling / moving around / reconnecting everything in the lounge so will see if I can resolve the amp ultra usb issue then.
It's surely something to do with everything else plugged in there as when I had the amp ultra in the office in a much simpler setup I had no such issues using usb with the rme or k11 r2r.
Well, not quite the quick, consise post I imagined when I started..