Hey, Alex. Some thoughts:
Video Quality
From what I've found on the internet, the 2600 doesn't support 1080p at all. It will do 720p or 1080i, but that's the limit. So, for video quality from blu-ray and cable, you're better off running the HDMI connections straight to the TV.
Audio Quality
You can run the TV optical audio cable to the 2600 as you mentioned, but (and I forget which Panasonic TV you have) most TVs will only output two-channel audio through that output when using an HDMI source. Does the cable box have a digital audio output? If so, I'd suggest connecting it and your Denon to the Yamaha using the digital audio connections. That way, you'll get full surround sound, as the digital audio connections will support all of the audio codecs from those two sources (neither will do the newer lossless codecs like DTS-HD, so you don't need HDMI from them for audio).
You could also get an HDMI splitter like one of
these that would let you run audio/video to the TV and audio to the Yamaha, just in case the Denon and/or cable box make you go through the menu system to change the audio output from HDMI to the digital audio jack. Well, or just get a new receiver.