Headphones

Well for me it's like this sorted by Priority:

• Y/N - Do they fit comfortably and fully seal?
• Tuning (after EQ)
• Aesthetic
• Distortion below 1-3%THD above 150Hz at max listening Volume
• Other Features?
• Tuning without EQ, if I wouldn't EQ it'd be at 2nd pos.

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Aside from tuning what parts are giving advantages? The housing, the drivers, the materials used?

They all impact the Tuning. But assuming that the Tuning stays exactly the same are the Main differences, that i know of:

• How easy is it to produce
• Price
• Aesthetics

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Example, the truthear nova ($150) is tuned nearly the same as the moondrop variations ($520). Does that mean they would have the same sound quality?

No, they measure differently so they will sound different, but close in "style".

• The Variations has a Stronger Bass boost and more treble(12+kHz) compared to the Nova.

Simplified is in the Literature a 1dB defined as the smallest difference in sound level the average person can hear. (50% of participants said they hear a difference)

-> if the difference in measurements by a Coupler is below 1dB you can consider 2 IEMs to sound the same assuming no other factors change the FR at your Eardrum. (for example Driver Stiffness, Impedance of the Eardrum,...)

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Distortion below 1-3%THD above 150Hz at max listening Volume

Currently is the Science not fully in agreement how big/low the Distortion should be and how it should be measured. (Intermodulate Distortion vs. THD vs. ...)

But in general seem 1-3%THD to be a good indicator at what point distortion is audible.
You can scroll down here and see the results of different studies.

I added the "above 150Hz" since i can't hear Bass Distortion as much as distortion in higher frequencies.

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And to add onto the comment u/LevanderFela wrote:
That sounds rather dismissive - soundstage, instrument separation, etc. matters too.

Yes, all these aspects matter, but they can all be explained by looking at the measurements and design of the IEM.

Imaging / Soundstage:

• How is the Magnitude Response compared to your HRTF (Head-related-Transfer-Function). If it follows your HRTF for a Signal from a certain direction it will trick your brain into thinking that it's coming from that direction. (https://en.wikipedia.org/wiki/Head-related_transfer_function)
  But one big Problem Remains, the IEMs are in an enclosure and therefore it can only partly simulate the HRTF, not use it. That's the Reason why imaging/soundstage with IEMs can't fully simulate Speakers in a room.

Instrument separation:

• This can be explained by auditory masking that changes cause of the tuning(https://en.wikipedia.org/wiki/Auditory_masking)
  For example, if you boost 300Hz it will sound "muddy" cause they mask nearby frequencies.

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So going back to your Main Question:
No, but FR-measurements of the Tuning are currently the best way to predict the perceived Sound Quality / if you'd like a IEM or not.

Fit and Seal can only be tested by buying the IEM.