I finally solved my problem after searching AVS forum. I have an old Yamaha receiver..HTR-5840...but I am sure this procedure works for numerous models. The main stumbling block was how to program the remote for turning on and off the Receiver.
Following the standard xfinity instructions for programming your remote worked ok for controlling volume, mute, etc, however, turning on and off was a problem. That is, using the AUX button and the 30176 setup code I was able to program the remote for controlling volume,etc.; however, programming the remote for the "discrete" on/off functions required the following additional procedures (thanks to Kupakai 1/13/08):
1. Press and release AUX
2. Press and hold setup until the AUX light blinks twice
3. Type 994....I believe this places the remote into the "discrete" programming mode.
4. Press and release immediately (do not hold down) SETUP
5. Type 00248
6. Press and release POWER
The remote should blink twice. That should set the combination of first pressing AUX then POWER to power on the Yamaha Receiver. Note, it should also power on by pressing the All On button.
Since the Yamaha remote uses another button to power off (i.e., the standby button), we have to assign another button on the Comcast remote for powering off. Fortunately, this can be accomplished by using a combination of two buttons (i.e., SETUP and POWER) on the Comcast remote.
Follow the same procedure as above, however, now use 00246 in step 5, and press SETUP between steps 5 and 6. Powering the receiver off is now accomplished by pressing and releasing AUX, then SETUP, then POWER. Note, turning off the receiver will not be accomplished via the All On button.