SB6141 Modem - after firmware update have just one upstream channel - had 4 bonded channels upstream.
I tried to resolve with telephone support but the person transferred to a level II tech who had no idea what I was talking about. He said his supervisor has told him there is no one else at comcast to answer/resolve this question. Apparently the blast 75 profile/config file for the upgraded firmware is not telling the modem to use 4 channels.
the other possibility is that comcast's CMTS is not allocating. Since CMTS has something close to 5 to 1 more upstream channels available than downstream it can't be a lack of channels.