It is my understanding that when tubes are on, they are always running full tilt. If the volume control is set very low, the signal coming into the amp is attenuated, and therefore, you have a very small signal on the grid. And a very small signal on the grid results in a small signal out of the amp. If the volume control is set to the max, the signal coming into the amp is not attenuated at all, and therefore, you have a larger signal on the grid and a larger signal out of the amp. So the volume control has no effect on the tube itself. It only affects the size of the signal introduced into the tube for amplification.
Going back to using a preamp and an amp, both with volume controls, it is best to turn one all the way up to eliminate any resistance in the signal path. And use the other to adjust the level at the headphone jack. I use the analogy of a stereo system with a pre-amp and a power amp. Typically, the power amp has no volume control. Controlling the level is one of the functions of a pre-amp. So I crank the volume of my "power-amp", that is, the LD1+ my headphones are connected to, all the way up and I control the level with the Audio-gd. However, similar to ILM2 with his LDIII, I hear a little ground hum when it is maxed out, so I back it off a tiny bit, to eliminate the hum. My guess is there needs to be a minimal level of resistance in the volume circuit in order for it to operate properly.
My advice for ILM2 is to set the Crack to the maximum, as this is your "power amp", and control the level with your LDIII. If the Crack hums full on, just back it off a hair.
Anyway, this is only my opinion. It works for me. But of course, I know just enough about this stuff to get myself in trouble... but not enough to get myself out of trouble.... lol