I have had the Matrix I Pro 3 for about a day and I can confirm that the headphone output is un-usable for low impedance, high efficient headphones. I tested with pair of T5p on both the 4.4mm and 1/4" and they sound awful. Which is a very different experience from others on the thread. But, I think I understand why.
The T5ps are low impedance at 32 Ohms and high-efficiency at 102 dB efficiency which means they need high current, but not high voltage. The Mini - I's headphone output provides lots of voltage but low current. For Dynamic driver headphones, normally lower impedance means high efficiency. But, that is not true for planars. They need both high current and high voltage. I recently watched a video from Abyss () where they suggest the AB-1266 (~50 ohms and 88 dbs) does ok with less current, if you give them enough voltage. So YMMV on different planars.
Fortunately, you don't have to spend thousands on a decent headphone amp for high efficiency / low impedance headphones. The bad news is the RCA outs on the Mini-I are not as good as the balanced output. The Mini is only using half of the amplification circuit for the single-ended outputs. So, you lose a lot of the sound stage going single ended. I am going to try and connect the 4.4mm output to a portable balanced amp and will report back.
From my understanding, the output stage of an amp is where they do things like create more current (lower the impedance), sum a balanced circuit for single-ended outputs, and add an analog volume pot (the Mini-I has digital volume). All of which is expensive to do well.
So, what keeps the cost down on the Mini-I is the lack of a high quality output stage. That is fine if you're using it with the right headphones or as a power supply/streamer/DAC. All of which are high quality. You can also use it as a pre-amp if you are ok with ESS' implementation of digital volume control. IMHO, I think it's actually better than the analog pots found on most mid-fi equipment.