Thank you very much for your reply, RythmikAudio!
I just phoned the soundbar manufacturer's TechSupport, and asked if it's SUB out is filtered above 80Hz.
They weren't sure precisely what I meant but explained that all that gets sent to the sub is what's below 80Hz. Which I'm thinking means a Yes to the question. (In which case it sounds like I've already got the F8's Crossover and LPF settings as you recommend.)
Since yesterday I arrived at the F8 Volume setting I have now (3 ticks left of 50%) by finding what 'sounds right', I don't understand why I would want it to be "6 ticks to 8 ticks lower" that that (than what sounds right). I must be missing something.As for the soundbar being too weak, you need to set the volume control knob on F8 from what you have now to 6 ticks to 8 ticks lower.
This will bring up the relative gain of all other channels over subwoofer. My personal preference is center channel should be 2-3db higher because the location of center channel is dead center and that seems to make the perceive loudness to be lower than other channels.