Separate subwoofer amplifier with remote. What would work for stereo setup?

If you used Kyle MacLachlan as your avatar it's possible he would be thought of as a Paula. 

Ouch. Remember, my name is a killing word.

Haha I'm just saying his hair in Dune is epic. That amp makes sense to use. Do you use any of the EQ functions or other settings to good effect? 

Check out the OSD Audio’s SMP500DSP or Velodyne SC-600D, both come with remote control.

I use a Velodyne SMS-1 connect to Sunfire stereo amp to power a pair of 15" passive sub.

There is no stereo with subs. You can hook em up that way. You can position and pretend they are stereo. But it is all mono.

Sorry not correct I have two subs, running stereo, each on the outside of my mains, they run individually off the line level left and right channels. You can clearly hear stereo subs when the recording dictates for it to be heard (especially orchestral works, tympani in back right channel double bass far right). CD has over 100dB of channel separation all the way down to 0hz. 
https://cnx.org/resources/e9e985c5b348f2d44cfbc3947900dc96475ba08c/graphics1.png

Different story with vinyl, as it has at best only 30db chanel separation at 1khz, and almost no!! channel separation at 20hz (in other words mono)

Cheers George