2 subs connected to a 2 channel amp?

Yeah, dual subs can often become a connection  issue. That's why I like my Parasound A21+, it has right and left "loop outputs" just for them. It does make a difference vs. make do connections. 

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.