Slingbox's are ment for higher quality HD streaming.
If you want analog through coax, you can always hook up an old vcr to it, or purchase an analog tuner.
There are very old slingbox models that have the analog tuner that sell really cheap on ebay. I'd just pick up one of thouse.
That way you have two streams of TV if you wanted it. 1 for older analog and 1 for newer hdtv.
Thats what i do..