Limiting the scope to your original questions, I think it really depends on what you want out of your system. I went from using the power amp from a Denon 3030 to using a Sunfire Cinema Grand and the difference wasn't huge with low quality input, things like VCR tapes. It was nicer, and it was there, but overall the change was somewhat disappointing. CDs improved nicely, but I wasn't stunned by their quality. On the other hand, high quality input like DVD sound tracks took a huge leap, and went to theater quality. I don't have a turn table hooked up, but I would imagine it would also be tremendously improved. The sound from a good movie soundtrack still makes me sit up and take notice, just because of how good it is. So, I would suggest that the decision depends on what you want to play. I would also factor in whether or not you will be upgrading the rest of your system to take advantage of the quality of the amplifier. Feeding a Sunfire from a Denon will never really take full advantage of the Sunfire.
Since I play a lot of DVDs (I work for Netflix.com) I would never go back. On the other hand, if I was planning on keepintg the Denon front end and didn't play very many DVDs, I would probably stick to the Denon power amp.
I like the Sunfire Cinema but I haven't tried the Signature. Generally, I think they sound pretty close to each other. Do you really need that much power?