Roon? does it offer subwoofer output volume control?

I will assume that the SVS is connected to the line or speaker outputs of the Hegel.  If so, no upstream device can control the SVS.  Roon can only control it if the SVS is connected to it, not to an amp.

simple 2 channel

bluesound node

svs sb3000 (seeking to replace hence the need for the remote volume feature)

hegel h95

It depends on the rest of your system but Roon definitely does support subwoofer level control and, even, bass management.  I have used it for stereo, 2.1, 5.1, 5.3....

What is the rest of your system and how is the sub connected?

Roon offers comprehensive EQ but it does not have a 3rd output channel

If you want something that has a separate sub out and comprehensive DSP, try the miniDSP flex.

Don't think so.  I use it and if it has that I've never seen it.  In general roon offers 2 channel DSP.  Remember roon processes the digital signal upstream of your streamer which then goes to your streamer which feeds your DAC.  No place for a sub signal to branch off that I can see. 

If you are an SVS fan, get the Pro series which offers control via an ap on your phone.  Highly recommend.

--Jerry