Consistently slow speeds

I have been working on this for about two weeks now, and last night I finally got a service call scheduled, but I don't think it will fix the issue.  I noticed lots of Cox reps helping on the forums, so I thought I'd give them a try.  

For the past two weeks I've been getting very consistent speeds of 12 MB down.  A week ago, I moved up from Premier to Ultimate hoping that would boost my speeds up to normal, but they've stayed a consistent 12 MB down.  My upload speeds went from 15-20 MBps to 30-35 MBps after the change, but download remains unaffected.  I don't think it's a bad line, I think I'm coded incorrectly on the switch.

I'm using a SB6190, with 32 downstream channels, and I used to get around 50 MB down with this same setup prior to two weeks ago.  

The modem looks to be operating within tolerances...  I'm an IT guy, but I don't know a whole lot about the co-ax stuff.  But to me, it looks like I'm being throttled, not a bad line.

Also, Cox has done a modem re-provisioning, 3 modem refreshes, and the modem has been rebooted a dozen times.  All the normal first-line troubleshooting stuff hasn't helped anything.  My speed measurements have been done from the router directly plugged into the modem as well as other machines on my network on wired connections, using speedtest.net with multiple servers, Cox's speed check tool, and my router's built-in speed test.  Every single one is a very consistent 12 MB down.