Well,
i was impressed after the 200 hours burn in with stock app, without disabling sound adjustments or trrs,
may be because it outperformed my other daps (dx50, fiio x5, shozy alien), while these daps are much cheaper, i have seen others compare it to more expensive ak daps like 240 and still give it the upper hand, but anyway its a matter of preference.
and i didn't try out the a15 but if its so close to zx2, i guess we have a killer dap at such a low price.
honestly disabling the sound adjustments or trying third party apps didn't do it for me, but trrs did.
Also, i never touch this significant refinement when pairing an iem with totl dap against totl dap + totl dac/amp, i find improvements very minimal.
so pairing an iem with either zx2 or zx2 + hugo, i would definitely go with only zx2.
but with an inefficient headphone thats a totally different situation, and iam not talking only about the volume, but big improvement there.
thats just my experience ..