Glad to help. When I read your inquiry, I was surprised at how similar your situation was to mine.
Yeah, there isn't a great difference in sound quality between the S4 and S8s, they are just built for different purposes. The S8 is for large rooms or for people looking for full range mains.
So what do I power my Sigs with? Well, first understand that I believe and it has been my experience that sound quality is most greatly affected by the the recording, the speakers, and the room. When it comes to front-end gear, I think you hit the law of diminishing returns pretty early. That is, I don't feel there is a significant difference in sound quality between a high end receiver and separates.
With that said I currently own a Denon flagship receiver, the AVR 5800. To my ears, the sound is clean, balanced, dynamic, and the soundstage is expansive (with the right recordings). However, and at the risk of sounding hypocritical, I will be replacing my Denon with separates later this year. I love this hobby and I'm willing to spend a lot more for a small gain. More than likely I'll go with Rotel, B&K, or Anthem.