I also had a model 970 with a 7075. It was a great alternative to using a receiver. I felt the sound quality was an improvement over using a receiver. At times, I did find the unit glitchy - especially with any optical inputs. Video switching worked well (and there was a shorter delay than the 976). It worked for me and then I lent it to a friend who also used it. Between using the 970, I have used a few receivers as preamps - Denon and Yamaha both were models 1 down from flagship and they worked well. I am now using the 976. While I was more than happy with using receivers as preamps, the 976 was still a step up in sound quality. The interface of the receivers was always better than the 976 both in front display and OSD/menu. The delay in switching sources is about the same. The receivers offer far more options in terms of connectivity. If you want a work horse type pre/pro, go with Outlaw. It is simple to use with better than receiver sound quality. While I prefer the sound of separates, if the 976 stopped working, I would go back to a Denon or Yamaha as a preamp.
To answer your question - sound quality, connectivity, interface, budget and personal preference are the considerations between a dedicated pre/pro and all in one receiver.
If you go with the 3600, I would be curious to read your thoughts.