Using the control on a subwoofer makes it a lot harder to have a smooth transition from the mains to the sub, than it is when using a receiver's crossover setting.
Most of the time the sub's control is a low-pass filter, and not an actual crossover (a crossover being a low-pass paired with a high-pass).
I find it much easier to set up the subwoofer, by totally disregarding the numbers, and doing it by ear (or SPL meter). You'll need to find a low frequency sweep sound. Play that on a loop. Start with the subwoofer's filter setting rather low. There should be a dropout in sound pressure level between the filter point and where the main speakers take over. Gradually raise the setting on the sub until you get a smooth frequency response through the whole transition range. That's where it should be set regardless of what the number says.
When you say "the receiver's crossover setting".... you mean the tone control? i.e. "bass" knob? (I'm using a very vintage receiver btw..)
thx
