2 subs connected to a 2 channel amp?

If you use the sub out of the node and use the volume control of the node you would have to you the pre-in on the r300 bypassing the preamp in the r300.

Otherwise, the volume control of the r300 would change the volume of the speakers but not the subs. Also, the subs would only work with the node and not any other source.

Before you buy any subs I would call the Manufacturer and see if the high level input will work with the r300. When I called Hsu Dr. Hsu actually answered the phone. Very happy with my ULS-15's! 

If you purchased a pair of REL subwoofers you wouldn’t have had to deal with this.

I use 2 RELs with the cables joined in banana plugs (the positive joined, and the 2 leftover negative leads into left and right negative terminals) with the main speakers using spades. Subs of course are mono which does not matter...Works great for years with various amps.

@wolf_garcia are you saying that you have 2 subs connected in series across the Right and Left negative speaker terminals? Such that each sub’s negative terminal is connected to the negative speaker terminal and each positive sub terminal is connected to the other, i.e. ‘joined’ by the red wire? 
 

Or, are you ‘joining’ Left and Right Speaker positive terminals to each other before going out to the positive terminals of each sub? (I.e., a parallel connection)

Trying to figure out how you’re getting three wires to do the work of four.

Jeremiah