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

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. 

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

@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.