Computer is in one room, audio system in another

[xxx (Banned - toxicity)]

Will having the cable company install another modem upstairs at my DAC and system location allow me to keep the computer/hard drive downstairs and gain any advantage of having a modem hardwired to the DAC?

Short answer is no. Generally speaking, one can only have one modem per network.

OTOH, any competent electrician can snake an Ethernet cable thru the walls to where you need it.