It will takes months to fully burn in. It can be used straight out of the box once you put valves in, but it can be a tweakophile's wet dream if you want it to. Suugest trying 101ds and 300bs to get different sounds.
As for using it as a pre, I prefer other preamps in the chain, though directly it sounds cleaner and faster. I prefer the weight from other preamps, even when I can hear more distortion. That said some of my friends preferred it direct, so that is more preference.
Regarding the volume control, I just use it for DSD on and off, and at the start to put it on and bring the volume up to Zero. So no trouble for me.