I just upgraded from a ATH-M40x to the Hifiman He400i. The M40x were obviously easy to drive through the Fiio Q1 MkII (borderline unnecessary even), and I assumed with the power output that little device was capable of that it would easily drive the 400i's as well. Maybe because they are Planar Magnetics they need more power? I listen to my new headphones with the volume knob turned all the way to maximum and it could still be a tad louder...
It's not simply because the HE400i are planars per se, it comes down to specific driver design and sensitivity, which is one of the parameters that has to be considered along with response and impedance. Some headphones have to go with a flatter and broader response, ie, the HE400i; some older dynamic drivers have to go with high impedance to gain primarily higher sensitivity, like the HD6xx line, but current tech doesn't rely on impedance as much as back then to get decent sensitivity coupled with the prevalence of higher output, low output impedance amplifiers allowed for the 150ohm HD700, HD660S, and HD58X.
Another example: HE400S is a planar magnetic headphone but its sensitivity is only 1dB/1mW lower than the M50X, but its response curve looks more like what the HD600 or K7xx will do (albeit with lower impedance that doesn't require amplifiers with good voltage performance without sacrificing sensitivity as the K7xx series does, ie, you can run the HE400S straight out of a Fiio X1 and still get enough to damage your eardrums in no time). The HE400i by comparison is 6dB/1mW less efficient, which means that to get to the same output level that the M50X are playing at, you'd need roughly 4X more power (each 3dB increase requires double the power).
In short it's like taking a Ford Duratec 1.6L engine that was a favorite engine swap into a 1990s Kia Pride 5dr hatch (Ford Fiesta in other markets) and then putting that engine into a Genesis G70 or Kia Stinger.
That said, did you try the Q1 Mk2 on high gain? If it doesn't audibly distort at the level you listen at then you may not necessarily need more power. It's just that either as a DAC-HPamp or DAC the line output from the DAC stage is just 1.2V.
...and it seems like the mids are slightly recessed.
Going louder might not solve that beyond just making everything louder the same way people crank up the HD600 or K701 to hear the bass a little bit better, but that won't change how the HE400i's response curve really goes from a plateau from 10hz to 1000hz (which is what people who like this like it for) then goes into a wide valley between 1000hz to 6000hz.
Could it be an issue with the DAC/AMP combo or should I just do another upgrade? Obviously to something more desktop like the M&M combo...
Magni will have no trouble providing power that can hit max dynamic peaks that proper recordings have.
The problem is that that's only half of your problem, the other is the response of the headphones. If you really don't like how hollowed out the midrange is or even just accepting it in favor of having everything nearly perfectly flat from 1000hz down to 10hz, ie, not having bass that just rolls off from a wide peak since very generally human hearing, physiologically and psychologically, has a tendency to favor the midrange and then many have a sensitivity to the midrange-treble transition range, you might be better off just getting a headphone that doesn't have that range that normally would be EQ-ing out human hearing bias even if it means a narrower flat range.
In short you might be better off getting the HE400S. On the flipside, if you upgrade the amp, you can try other headphones without getting limited by the output power on the Q1 Mk2. Just note that the Magni3 still has just less of a tendency than the previous versions to distort in a manner that makes the top end more assertive, and since by 6000hz the response is back up, there's a chance that the thinner midrange coupled with the stronger (by comparison) response at the top end with the Magni3 potentially making that range only a little bit worse will sound even worse to you. So if you do go with the amp upgrade, maybe try to get something else; like the AudioGD NFB-11, Gilmore Lite Mk2 or Meier Jazz FF, maybe the Asgard or Arcam RHead. If you get the RHead, Gilmore Lite Mk2, or Jazz FF, maybe just use the Q1 as the DAC first, and if it doesn't get loud enough or you just have more cash later, try a DAC that has a 2V line output (the Q1 Mk2 only has 1.2V).
