AK100 + Momentum: Works well. I'm not a big fan of the Momentum, a bit too bass-heavy (but I realise a lot of people like that) & treble is very shelved (and that's coming from someone who is usually sensitive to treble) It's easy to drive, and since one of the good things about the AK100 is the rocksolid output up to max. level, no problem there. The AK100 is generally also quite neutral (some consider that cold or "un-musical"?), so it keeps the bass in check without altering the treble noticeably. In short, in my opinion, it makes the Momentum as "audiophile" as that headphone can be, in a decent portable package (that alas cost considerably more than the headphones themselves) If the bass is missed (really?), another good thing about the AK100 is a decent EQ which can be used to bring back some of it.
AK100 + Tzar 350: My favourite IEM combo, as good as some of my favourite fullsize cans (I'm not much of an IEM guy). Here's an excerpt from a PM I wrote about the same subject:
"If there is a consideration, it's the sound signature of the Tzar 350, not so much the AK100. And how well that works for you, I can't say. It is on the brighter side, and people have had issues with sibilance. Funnily enough, I'm quite sensitive to that too, and have had issues with that on a recent headphone-testing spree of sorts, but not with the Tzar 350s. My one niggle with the Tzars, incl. on the AK100, is more the somewhat weak bass (and I'm not a basshead by any means). So depending on the music, I may turn on the EQ and bring that up a bit (and another thing they got right with the AK100 is the EQ implementation) In comparison, my other "higher-end" IEMs, the Shure 535s, require adjusting the treble down and more... I personally much prefer the Tzar 350 (on any amp), and the Shures will go back on the market.
The AK100 is quite neutral to my ears (again, relative to my other amps), which IMHO explains why for some people it does not work so well combined with the notable treble and somewhat recessed bass of the Tzar 350s. But -I- personally find them overall well-balanced which I prefer over a headphone (or IEM) which excels in one particular area."