i did a short a/b listening session with the HDP vs the ibasso combo. both using the USB to single ended t50p.
HDP bought used. seems like low hours.
ibasso mine. low hours.
ibasso pros:
better bass impact
more dynamic, fun sounding
brighter
full sound
better clarity
option for balanced
ibasso cons:
two separate units
cant be considered neutral vs the HDP
slightly inferior build quality
it really depends on the headphones and the person's personal sound signature preference. the ibasso fits my signature better. i dont havve any other headphoen at the moment besides the T1 and i dont have a single ended adapter yet. its in the mail.
HDP: build quality feels absolutely awesome. decent weight to it, looks great, and works well. the sound is more neutral. bass is good but less impact than the ibasso. overall the sound sounds balanced.
overall, i dont feel the HDP performs as an amp above its price point as others may think. not that i have a ton of experience or anything but i think it was just decent. for 449$ new its a nice 2 in 1 option but its not the signature i want. but if i was on a budget and couldnt afford the ibasso combo, this would probably be the unit i would go for. *disclaimer: i havent tried many of the dac/amp combos like the pico, d10, etc.
if i could keep one, id keep the ibasso hands down. im building my full sized system so i have to consider selling the ibasso combo even tho i dont want to.
these are my humble opinions.