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Motu M6 killing sound stage & quality for headphones?

The M6 can only supply 3V into headphones but only in 300ohm
I expect the headphone amp section of the M4 and M6 to be the same.

HE6SE = 100dB/V
The Motu M4 can deliver 24mW in 32 ohm = 0.77V (24mA) and so I suspect 1.4V in the HE6SE.
So 103dB peak SPL. That seems loud but it isn't.
It means that when a DR10 recording is reproduced the amp already reaches limits at 93dB average.

I recommend to use an external headphone amp that can drive the HE6 with ease if you want to use that one.
 
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This would be a rare case of delivering on the specs. As even small deviations change perception, you need to calibrate (measure and adjust).
Easier said than done. Could you kindly explain to the OP, how they may achieve such a calibration, i.e what tools to acquire and how to use them, or point them to resources on the web to learn how to do this.?
 
The M6 can only supply 3V into headphones but only in 300ohm
I expect the headphone amp section of the M4 and M6 to be the same.

HE6SE = 100dB/V
The Motu M4 can deliver 24mW in 32 ohm = 0.77V (24mA) and so I suspect 1.4V in the HE6SE.
So 103dB peak SPL. That seems loud but it isn't.
It means that when a DR10 recording is reproduced the amp already reaches limits at 93dB average.

I recommend to use an external headphone amp that can drive the HE6 with ease if you want to use that one.
I also made the same mistake to think the OP was using the M6 Headphone amp. If you read through the OP's configuration, they are already using an external headphone amp, a pretty decent one, so loudness should not be a challenge. The external headphone amp in use is pretty powerful to drive almost any headphone to deafening levels.

The issue here is matching levels from one audio interface to another, so as Joachim Herbert said, one or two posts behind this, the OP now needs to calibrate both audio interfaces, to match their levels.
 
...or point them to resources on the web to learn how to do this.?
Setting up a level matched comparison is tricky* indeed. A viable alternative is described here. It consists of digitizing the output of the DAs under test, level matching them and feeding them into an abx-tool. All you need is a really good A/D (which comes cheap these days). Errors may be introduced by the A/D process, but these should be minor.

Taking this route you may evenuse technical means to show the differencial. Or you may just look at the signals to compare them.

* Several problems, among them:
  • Setting the interfaces to the same output voltage would seem simple, but might degrade their resolution, in case they have no analogue output control.
  • Setting and measuring output voltage at the headphone amp would require a volume setting that is continious and highly reproducible.
  • Time to switch between sources will probably bee too long.
 
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So I recently got my first audio interface to do some recording, but in the mean time it has become a full time DAC on my desk setup.

My signal chain is now setup like this: PC -> Audio Interface -> outputs 1 & 2 TRS to XLR balanced outputs -> Topping A30 Pro XLR Inputs -> Hifiman HE6se

My first try was with the Scarlett 4i4 4th Gen, and it sounded absolutely wonderful. The sound stage is so good and clean, it's the best the Hifiman HE6se have sounded for me. (Maybe just with the extra headroom of have balanced inputs).

But, I needed to actually record 4 mic inputs, so I needed a more capable interface. There wasn't a 4th gen of the larger scarlett series yet, so I bought the MOTU M6. Plugging the MOTU M6 into the same signal chain above, but I was pretty disappointed to hear the sound quality is actually quite a bit worse. The volume is there, I can pump them to painful levels but it's like the clarity is gone, and the sound is muddier, boxier, less transparent.

I am not an expert at this so I am probably doing something wrong or misunderstanding some audio concept. Can anyone give me advice? I can't keep both, and I can't really go back to the audio quality pre-scarlett, but I also need the extra inputs, so I kind of stuck in a bind. If I can get MOTU working with the same quality I'd be really happy!
That matches my personal experience.
I've had a few audio interfaces, but I always found the sound performance to be worse, especially the built-in headphone amplifiers. RME and Violectric are in a completely different class.
Only the Focusrite Scarlett 8i6 really surprised me in a positive way.
 
Ah right ... Topping A30 Pro
I also made the same mistake to think the OP was using the M6 Headphone amp. If you read through the OP's configuration, they are already using an external headphone amp, a pretty decent one, so loudness should not be a challenge. The external headphone amp in use is pretty powerful to drive almost any headphone to deafening levels.

Can't really explain the perceived effects in that case.
Would need some serious ABX to verify what is perceived. Not easy to do.
 
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