Oh, there are definitely more than a few choices available when you are looking for an audio source to pair up with A12/U12 (you can look at the index of my head-fi reviews
HERE).
First you have to figure out your budget, and then decide if you want to stream (Spotify, Tidal, YT, etc) or only care about local storage. If you need streaming, you have currently fav choices of FiiO X7 (w/AM2 upgrade it rocks!) or Onkyo DP-X1, or getting something like Mojo or another usb dac to pair up with your phone. If you don't care about streaming, your selection really opens up from mid-fi excellent budget choice of $180 X3 2ng gen or the latest iBasso DX80 (excellent touch screen, dual micro SD and optical out) or Cayin N5 (dual micro sd, 2.5mm TRRS, solid build) in $350 price range. Another one which I'm waiting to receive for testing next week, $300 L&P L3 with balanced output, touch screen, and complete integration to use with your smartphone as portable usb dac. Two of my currently favorite sources are X7 and PAW Gold. Actually, PAW Gold is mind blowing (and currently on MD for $1400) and in terms of sound quality I found it better than AK120ii and AK240. Never tested AK380, but talked to a number of people who consider PAW Gold to match AK380, though being a little inferior to AK380 Copper version. One thing that I really like about PAW Gold and DX80 pair up with U12 - a very impressive soundstage expansion and spacial details, both of them excel in that.
But here is a thing, before you start looking into a new source, the #1 suggestion is to upgrade the cable, as you have read from many others who shared their upgrade impressions in this thread. I'm listening to U12 w/TWau straight out of my Galaxy Note 4 and these have quite an impressive synergy. I can't speak for other cables since I only use Linum and Whiplash. Linum BaX doesn't pair up well with U12, upper mids get pushed back and not the same level of soundstage expansion in comparison to TWag/TWau. Either pure silver TWag or gold plated silver TWau provide a very noticeable soundstage expansion to take the 3D holographic effect to the next level. With these two cables everything scales up in sound quality from improved layering to better retrieval of details, and overall sound is tighter and more dynamic. The difference is that TWag sounds sharper/crispier while TWau smoother/organic - subtle difference but I can hear it (I spent last night 2hrs listening and cable switching).
I know that not everybody believes in cables or hears the difference, but in this case the difference is more pronounced in comparison to some other IEM cable upgrades I have tested in the past. In my opinion, if you get a good deal on A12/U12 with a promotional discount, take that saving and invest into a cable upgrade. And when you are looking for a new source, don't assume that you will have to stretch your budget to $3k+ in order to squeeze every ounce of A12/U12 performance. Again, I'm speaking from a perspective of these 12-driver models where you don't need a super revealing analytical source. These are smooth detailed TOTL monitors that actually have a great synergy with neutral/warmer sources.