I have a WA-2 and a LF332C tube amps so I think I have an idea what the D18 is capable of doing (which is why I am in no hurry to replace it). Sometimes I just like to change over to ss for the transparancy and calarity that you only get from ss. I upgraded the Matrix M stage and I think it pairs very well with the HD800 but I am looking for better ss. I don't think I will improve on my tube amps unless I get into the mega buck territory.
I use both outputs as I usually have two amps connected but only one amp active at a time of course. The difference in sound can pair with various amps better but in general the balanced output is slightly better, I do not find a huge difference. The I have auditioned many DAC's thinking of replacing the D18 but I guess I am hooked on the overall musicality of the D18 while still preserving most of the detail I hear in higher priced alternatives. Honestly I get as much difference with amp tube rolling as I do with changing many of these alternative dac's. Some of the dac's are hyper detailed at the expense of brightness that I don't care for. I have not listed to dac's over $2500 as I have my limits. Yes, clearly some I have heard are "better" than the D18 but only marginally so. Hard to compare all this in various dealer showrooms and get conclusive results.