There is no real full volume as it is a passive device. At certain levels (and also frequencies) the headphone starts distorting more and more, and you can say this is where the volume ends. You can safely plug it into your motherboard and decide if it’s loud enough and if you want to purchase a separate amp later.

LEMO FGG B-series should be above all. As many as 64 contacts if you wish so, is latching and many more features what you could wish for.

Assuming the same efficiency, the difference in sensitivity of a 1500 ohm headphone and a 600 ohm one is 4 dB, which is probably less than you think. Or almost 8 dB from a 250 ohm one.