[Sephiroth]

It's a good thing up to a point where noise injection power sort of swamps modem output power.

Fwiw, my 20 meg modem for years ran on 29 dBmv without problem; the street amplifier is < 80m cable distance away.

The upstream power level is commanded by the CMTS at the VM end. It commands the minumum power necessary (+ a small margi, I believe) for the CMTS to be able to make sense of what it is receiving from the modem. That can vary every 30 seconds or so depending on line conditions.

Other than that, it's what JB says; more attenuation at the street cabinet will raise the upstream power output by the same amount.