I had a brief chance to plug my SGT-DI into the FX return of the R100. TL;DR: the R100 and SGT-DI are fairly meh to ugh together. They are better off separated. Still, just brief initial impressions.
All impressions are without a cab sim engaged.
First, the SVT circuit with low to mid SGT settings were pretty decent. Not up to the SGT-DI as a headphone amp or recording device through the DI but is "rockable".
Second, the B15 circuit with any SGT settings. Er...not so much. The R100's 12 is tuned different than the B15's 15 and it shows.
Third, the Preamp section of the SGT-DI alone into the R100. Not bad, but not as good as my RB-110. What struck me was the way highs were handled. There is a different quality to the frequency range but especially in the treble region. The R100, by itself, has a very nice "exciter"-like quality to the treble region. It lends itself well to no grit to mild grit. I missed that treble quality when using the SGT-DI with the R100.