Total noob quesion here but I also cant seem to find a straight answer what is "better" Low gain or High. In my set up I have HD650 and HE400I on a Jotunheim and I have always done high gain.
Sonicaly, and I would guess this depends on the topology of the amp is it better to run higher volume low gain or lower volume high gain when at ISO volume? In this case on my Jot 2 im recieving soon, My gut tells me low volume high gain so the amp stays mostly in class A mode.
Thanks in advanced this is something I have been wondering about for a long time.
Call me a noob, too, because I have no clue what you mean by "ISO volume."
As for gain, it's a personal preference ... as long as the amplifier's volume pot is first in the signal path. I only mention that as a caution, because even though 99.9999% of headphone amplifiers put the volume pot as the first thing in the signal path, there was one crazy guy who didn't: NWAVGUY. His O2 amplifier cheated. You can reduce noise in the signal path if you place the volume pot between the gain stage and the buffered output stage. If you do that however, you run the risk of clipping in the signal stage if the source signal strength is too high. This can happen regardless of the power of the amplifier or the efficiency of the headphone, meaning it will clip all of the time on that source, no matter what you do.
Having a source signal too high happens more often than you might think (cough, cough, Schiit), so 99.9999% of all headphone amplifier designs put the volume pot, first. The only way an amplifier will clip when the volume pot is first in the signal path is if the amplifier doesn't have enough power to supply the load (headphone), which is what we all expect and understand from "clipping."
Anyway, sorry for that digression. Gain is actually a ratio indicating the amplification factor. It is independent of power, impedance, or load, but the effects can vary with all three variables, plus the impedance of the volume pot and the source signal strength.
Put simply, you want a gain with the amplifier, headphone load, and signal strength that allows you ideal volume control. Your personal range is probably dead silence, but just below the detectable, to the highest volume you can tolerate. If you're like most people, you'd want that range to be spread out from zero on the volume knob travel to the end stop of the volume knob travel. Some people are comfortable only going halfway. In fact, some amplifier designers may design the amp that way to make you think it's got a lot of power, because you're only halfway and you can't stand the volume. That's gain, though - not power. Chances are, the amp will clip beyond that volume setting. Similarly, you don't want a gain that allows you to max out the volume knob travel and it still doesn't seem loud enough. The amplifier might have a lot of power left, but your gain is set too low.
Gain is just that - adjusting it means optimizing the range of loudness levels to match the volume knob travel that you desire. Optimizing the range of loudness with gain means that you have the maximum control of volume levels possible with that particular source, amplifier, and headphone (also music file).