Here's where to get the information I'm using.

1) Sony codes. hifi-remote.com/sony.  The Comcast remotes can send any 12 or 15 bit protocol command.  For 20 bit commands, it can send 26.73 and 26.98, 26.250, 26.11, and 25.11.  Actually, it can send any ??.73, for example, where ?? stands for a number between 0 and 31, but probably only 16 and 26 have utility for your system.

2) The capability of the remote is shown at www.hifi-remote.com/jp1/lookup/.  A red button Comcast is a 1067BX3 JP1.3 remote, and the protocol you are interested in is Sony 12/15/20 Combo.

3) I get the EFCs from a program called RemoteMaster v 1.97, available free at www.hifi-remote.com/forums/viewforum.php, using this upgrade www.hifi-remote.com/forums/dload.php.  The upgrade is a little tricky to use, because it arranged the subdevices as 16, 73, but setup code 31758 is arranged as 73, 98. 

Now, in looking up these links, I noticed that Sony receivers use the 12 bit protocol, device 13 to send tuner commands.  You may want to try EFCs Preset+ 35445 and Preset- 34421.  It might work.