I am not currently using the 791 - I have the T+A SDV 3100 HV (with preamp) in system one, and SD 3100 HV (no preamp) in system 2. But, I’m using the Simaudio North 761 amp in system 1 and like it very much. I did audition the 761 amp vs the 861 amp directly. Yes, the 800 series is better, but they’re closer than many would suspect - the larger amp being more “effortless”.
Unfortunately, I did not audition their 791 vs 891 preamp/DAC units directly. I’m sure there are differences, but to the magnitude of the differences I can’t speak. Nevertheless, I did hear the 791 with the 761 and thought it was very good on both an absolute and relative basis. As I stated earlier - yes you can do better. But it’ll cost you in time, effort and certainly in money. I like my DACs better, but they are also a price point up. (So I make sure I tell myself they’re better!).
Sorry I can’t be more helpful.