First of all, I must say that this is the first headphones amp I ever own.
I am a student in electroacoustic composition and I have 5 years experience in studio recording etc., from the home basic stuff to proper studio gear (In my school we have enormous Genelec speakers, a big SSL console etc.).
Comparing same headphones (DT990Pro) with the HP4 output to the headphones output of the Audient iD14, one can surely feel a difference in volume, even though I do not need it.
Regarding the actual quality, I think - and feel -this aspect is improved, I do not have any actual data about it though.
So, is it worth to spend 100+ € for this? I do not know.
If you need more headphones outs, surely it is.
Personally, I bought this cause I had problems with 3 pair of headphones stop working without any reason (2 AKG 702 and one DT990Pro) so I wanted to understand if this was related to any piece of my electroacoustic chain.
Surely the "mono" button and the fact that you can pass through and send the signal to your monitor could be really useful (you have also a "monitor mute" button).