2 Subwoofer setting

Whatever sounds best in your room with your components. Not being cheeky, you just need to work at it a bit and see what sounds good to you. Sort out the phase first, then work out crossover and then volume. I had a Sunfire True Subwoofer Super Junior that I loved (but died on me after 20 years) but I don’t recall where I had set things. I did a lot of tweaking, sitting and listening, then tweaking again. It takes a bit or work but it’s not hard to get it to sound good. I just integrated two REL T5/x subs using their recommendations, setting phase, then crossover (I think I started around 20HZ above where my speakers cut out), then volume starting low and kept turning it up till it was too much then dialed it back. The idea is the sub adds weight without being obvious. This may not be what you are looking for but you really have to put in the time to sort out the integration you want. Room position plays a big part as well so you might need to move them around to get the best sound.

Ditto, room dependant.