I've just received this: Image link.

I'm going to use it to control the bass on my Edge EDB12A subwoofer.

What should I set the subwoofer's Bass Boost, Frequency and Gain ***** to, before I control the bass level via. the RCA level controller?

 

set the gain to match the head unit's RCA voltage output ( NO HIGHER )
and start with a x-over frequency of about 60hz..you can fine tune it up or down
once you start listening

I would leave the bass boost OFF until you have the frequency set correctly for
the enclosure... you might not need it ...
if the x-over tuning is too high the sub will play out of the tuning
frequency of the enclosure and cause weird noises or early distortion

I run my subs in the 40 to 63hz range (on board DSP that I can adjust on the fly )

 

I would really look into setting the gain with a Digital Multi Meter. For me, it really was the perfect way to set my gain. Then, I worked on the frequency and bass boost.

I have a LOC with a remote level adjustment, which is similar to your setup. When I set the gain, I set the **** all the way up to make sure the amp is receiving the highest voltage I can squeeze out of my stock/LOC setup lol.