Help with McIntosh Mx-120 Sub out

Hey experts, could use some help.  I have a MX-120 preamp that I have had for almost 10 years.  I have always used a CD player or streamer into the digital in of the MX-120 with a sub out running.  When I use the digital in the sub works fine, but now that I am running an up to date DAC with an analog signal out to any of the analog RCA inputs I get no sub out.  Unsure if there is something I need to do in settings or if the sub out is only enabled with digital in.  Any help or advice would be appreciated. Michael