Comcast STB recommendation for digital SD?

Yeah, as far as better SD boxes in the Motorola markets any of these would be better:
DCT-2500 (slightly smaller than the box you have now, different front panel button arrangement)
DCT-700 (Size of a cable modem)
DCH-100 (Silver / black - looks like a smaller version of a Newer HD/DVR)
DCH-200 (Identical in apperance to the DCT-100)

Out of that list only the DCT-2500 has a front clock display. In my area the DCT-2500's are a rare breed. I have a DCT-700 in the bedroom hooked up with the yellow, white, red RCA cables for better audio/video quality, and that runs circles around those older DCT-2000/2224 series boxes.

Each office (headend) may have different equipment.
Tell them what your issues are and see what is available!
If push comes to shove you can always threaten to go to Satellite!!

Tell them you want something new or newer than what they installed. We all pay Comcast enough they shouldnt even have those anymore.

I called them up and got a 2nd HD/DVR box they came out brought me an old used one when I asked for the new model with HDMI before they even came out. I almost didnt let the guy in when I saw him bring it to the door. I called back and have them ship me a new box cost only $10s/h called back and they gave my account a $10 credit.

Also you can check if you have a local service center close by and go there and see if they have a new model and they just swap them out for free just have to sign a paper or something.

those small 700 ones are nice. If you need or want a bigger box and dont need a HD box ask them if you can get the 100 or 200 since you have a 5.1 ss and need a optical port on the back of the box (even if you dont have a 5.1 ss might be able to get them to get you one of the newer models).