Adding a powered subwoofer RCA output to a tube amp

I have a Dynaco style tube amp that is really nice but I would like the flexibility of adding a powered subwoofer when I want the extra punch. I am still pretty new to audio but can read schematics and am learning fast. 

I am looking for some insite into the correct way to wire the connections. Can I pigtail directly off the line level input from the volume potentiometer in the amp and connect that to a pair of RCA jacks in the back of the amp (keeping volume control for the sub)? Should I ground the new RCA jacks to the regular main amp ground or should they be isolated? Should I connect a resistor between the new outputs to ground?

I have been trying to read and figure this out myself but really don't want to mess anything up and can't find a straight forward answer. Thank you very much in advance.
Why not just use a sub that has speaker level inputs?
I have a sub already with RCA inputs and having both options is ideal for my needs. 
With separate preamp and amp it’s easy.

Preamp to L and R line ins on sub
Sub L and R line outs to power amp(s) L and R inputs

This way the sub rolls off the mains, typically with a 2nd or 3rd order high pass filter around 80 Hz, and the subs take over below 80 Hz, depending on where the sub’s crossover is set.