Accuphase integrateds…are pre outs active full time to use for sub?

The pre-out / main-in switch in either position, the pre-out is active full time, you can connect it to a sub in the conventional way.

@russ69 would that cut the gain down since the pre signal is split in two?

You'll have more than enough volts to drive anything you want. 

@russ69 would that cut the gain down since the pre signal is split in two?

I think you can use a Y connector with one leg coming out of the integrated, one leg going back into the integrated and one leg going to the sub. 

what specific model number?

you can always get a sub that has both speaker wire in/out AND rca in/out.

rca in/out is best, it takes the load of low frequencies away from the main amp and those frequencies away from the front left and right speakers; both the amp and speakers get a break, lets you use a less powerful amp, perhaps then use tubes. sub sends everything except low bass back to the amp, via main-in.

speaker wires to/from sub does NOT give the amp a break. main amp still amplifies all the frequencies, including the bass. the sub takes the bass only, send everything else along to the front left and right speakers, so they get a break, don’t try to make low bass, thus stay cleaner doing everything else.