Presonus HP4

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).

HP4 works for me together with Evo4. Eve stands on top of it.
This is my first headphone amplifier.
But I want to start with how I chose this device from the extensive list of its competitors presented here. It was still an epic :) We all want cheaper, but amplification of headphones ... Well, what can be so complicated that it costs as much as 100 euros ?! However, you also don't want to get burned on savings by buying a cheap but unreliable device. And when you consider that there are many applicants for the place of your headphone amplifier, and there are negative reviews for each (and not only in the Thomann store), the choice becomes far from obvious and very difficult.
By creating a simple spreadsheet, you have diagrams of your preferences in front of your eyes. It was easier for me because it is very tedious to keep in mind all the points and reviews on them for each model.

So, the trick is to find out the percentage of reviews reporting
a) marriage in specific models;
b) any inconvenience in using the device;
c) insufficient quality of the work performed by the device
with the total number of negative reviews for the model.
Next, you need to see how 5-star reviews compare with 4-star reviews. And then their sum - to the sum of the other three stars. By comparing the resulting coefficients of the models you are considering, you will get visual structured information about what is best for you.
I hope this method will help someone (by the way, looking ahead, I will say that in this way I chose more than one purchase in Thomann, and I never made a mistake).

This is how I chose HP4 and am happy with it, like a pig on a rainbow))
pluses: case, power button (!), 4 headphone outputs, power (I have never twisted 250 Ohm headphones to maximum), mono / stereo, mute monitors ...
I have nothing more to wish for here.
He just does what he has to do. Yes, it sounds a little dishonest, the power button is on the cable, but guys, look at the price, the workmanship! And it's also good that this button exists at all - many manufacturers do not even think to please us with its presence!
By the way, by the sound. If you do not work with super-stars, or do not seek to get a Grammy, but are simply a home musician "for yourself", then the nonlinearity of the frequency response of this device will not hinder you in any way.

Verdict: buying Presonus HP4 is a very good deal in my opinion.