It's not really what iem will pair best with the dpx1 and xdp300r. Everyone will end up listing their current iem.
You should look into which iem will fit the sound you're looking for first.
A bit jaded/cynical, but probably accurate.
My 2 cents-worth (likely over-priced) is that it's all about matching source and transducers which synergistically support your sound preferences (couldn't resist wandering into marketing speak).
Seriously though, this device is fairly neutral, with excellent sound staging. Probably leans more towards 'analytical' than 'musical', so I presume some pairings might lead to a fatiguing sound sig (for example, I'd like to hear from someone who's paired this with Etys). Also, if you prefer a more 'intimate' sound stage, some pairings might be too big/open.
I've been comparing it to an Opus 1, and trying out some iBasso IT03s (along with my Frogbeats C5 & Aurisonics AS-2 CIEMs). I like the DP-IT03 pairing very much (now that I am using the 'right' tips with the it03s). VERY open and airy, with excellent detail and a big sound stage with very resolved imaging. Definitely not 'intimate'. Bass is also very tight and resolved, with a notable lack of 'muddiness'. I don't hear the pairing to be 'bass-heavy', but certainly hear a 'satisfying' amount of bass. I'd call the sound sig more 'bright' than 'warm'. Similar to the Onkyo with my Frogbeats, and much brighter than with my Aurisonics. In fact, overall, the pairing is very close to the C5s.
Since you mentioned the iBasso, I assume they're within your price tolerance. A point to consider might be cables. This device has two output jacks (3.5mm single-ended stereo, and 2.5mm TRRS balanced). The iBasso uses MMCX connectors, so (depending on what you already have in-hand) you might be looking at some significant expenditure for new cables, especially if you ant to go balanced.
I should mention that the IT03s are a bit large in the ear. Nearly as big as my CIEMs -- some folks will find that unacceptable.
