I also got the "check Sirius tuner" message but now have a connection. I have a Sportster 5, Sirius Connect Home Pro Kit SCHDOC1P and the Denon AVR-4308CI. The Sirius Connect Home Pro Kit has a female end RS-232 DB9 pin connector. The Denon also has a female end RS-232 DB9 pin connector. I went to RadioShack and purchased a male to male gender changer ($9.99) to connect the two. Still got the "check sirius tuner" message. I went back to Radioshack and purchased a null modem ($9.99) and connected it along with the gender changer. I now have music! All is working as expected. (BTW, also plugged in the analog R/L RCA plugs into the VCR/Ipod inputs). In the Denon menus I had to change Ipod assign to "none" and then Sirius to "assign". I then got a Sirius icon with the same controls as you get with the XM icon. This was after I had already completed the firmware upgrade. I then used the Denon web controller (went to a computer on my home network) and typed in the IP address of the Denon in the Internet Explorer address bar. I was able to go into the menus for the Denon sirius controls and assign my station pre-sets. If you have not used the Denon web controller I advise this to compete the set up of your music from sirius, vtuner, rhapsody, hd radio, internet radio, etc. Hope this works for you as well.
Home Theater 7.2: McIntosh MX150 w/ firmware to MX151, MC452, MC207, B&W 803 D2 Diamonds, HTM2 D2 center, CWM7.3 surrounds, JLAudio F112 x2 subs, Panasonic TC-P65ZT60 65" Plasma, Oppo 95, Sonos, Apple TV, Furman Elite 20 amp PFi x2, WireWorld Silver and AQ Cables/Cords, Salamander Synergy Four Bay Cabinet