Unless you have a lot of money, most of the hybrid systems are running stems with maybe one or two mono sources out of the DAW. I only monitor the output of the combined signals after they come out of the preamp and compressor. There are summing units which will let you monitor pre and post the summing. You could set this up easily so that a stereo pair(two mono outputs L & R) had their own separate monitoring chain.
From Sonar you just set up a stereo feed for each stem (basically mono left and mono right) into the summing hardware.
Understand the whole summing externally is limited with 8 out devices like Apollo, Lynx, RME etc. Where things get interesting, if you have the money, is at 16 out like the Lynx Aurora 16. You have stereo stems and mono outs which you can combine to the final stereo print, more options overall. The big boys will use mono outs for every channel on their mixer. I think past 16 outs however, the number of summing channels begins to have diminishing returns in a passive world. This is the reason high end desks usually have separate power sources and beefy transformers on their summing output. There are some very high summing boxes which provide this.