Will I hear a difference between the low and high resistance settings with the Audinst HUD-MX1? I'm driving a Sennheiser HD650 and I didn't want to dig into the device without having a true purpose.  The low setting is rated for up to 300 ohms, and the high setting is 300-600 ohms.  Obviously, the HD650s are rated at 300 ohms, so I would like to know if it would actually be beneficial to change the Audinst to its higher setting. Has anybody tried this with their own setup?

Anyone? I really don't want to ruin anything when opening it up for no purpose or benefit....

Just to make things clear: the MX1 output impedance is 5 ohm, regardless of settings.
 
The jumper is actually adjusting the gain factor of the headphone amplifier, it does not alter any resistive value on the headphone output.
 
The difference between the two settings:
- default gain setting offers 5V pp (peak to peak), for a 32 ohm load (or 800mW, roughly 100mW @ 300ohm)
- high gain setting offers +/-10Vpp,  for a 600 ohm load (no rated power, but it should be close to 200mW @ 600ohm and double that @ 300ohm).
 
Now, all these number don't mean anything by themselves.
Practically:
- the default gain is sufficient to drive just about anything. I can get decent levels of loudness with my AKG K501 (94db/mW) or the even less sensitive K240DF (rated at 600ohm and 88db/mW). It should have absolutely no problem driving the HD650 to quite loud levels.
The advantage of default gain is the larger operating range of the potentiometer and lack of clipping.
 
- the high gain offers 4 times more voltage than default. This setting basically provides more loudness should one need it, but brings absolutely no benefit to sound quality.
It is reasonable to use this setting on very low sensitivity cans (like the 88db/mW AKG K240DF), but anything beyond 96-98db/mW is just fine on the default gain.
The main drawback of the high gain is susceptibility to clipping and the short usable range on sensitive cans.
 
 
So, getting back to your original question (after a rather long winded explanation) I do not see any practical need to adjust the gain setting with HD650.
 
 
 
 

Thank you very much, that's all I needed to hear.

Thanks for clearing this up.

Digging this one up as I recently hooked up my HiFiMan He-4xx's to the good old HUD-mx1. Would the high gain setting make sense with these cans? The impedance is 35ohm, but the sensitivity is 93 dB, as quoted by Massdrop. And I keep coming across this "planars benefit from more power" thing.

So is it worth opening up the DAC? With the default gain the comfortable volume level is around 50-60%, but for some records I need to crank it up to 75-80%, and I'm not talking testing the limits, just comfortable listening.