Xfinity Gateway

I've been using my Nakamichi sound system without any sound interference. About 3 months ago, my 2 subwoofers started cutting out and making a popping sound. In troubleshooting, I was able to eliminate the interference by turning off the gateway so that I had no internet connection. I used a DVD to play a movie and there was zero interference. I mostly buy movies that stream rather than buying DVD versions. I attempted to continue to research possible solutions so I could continue to watch streaming movies using my Nakamichi home theater. I ended up arranging with Xfinity to start using my own modem. I was getting interference with my Netgear Nighthawk as well until I found a solution. All I had to do was to change the channel by logging into my Netgear IP address. I changed the channel to 11 and that worked; zero interference. Additionally, at that same time, my contract ended and my Internet bill jumped from $85 per month to $150 per month. I immediately contacted Xfinity and was able to get a new plan that put me back to $85, but there was a catch where I had to use their gateway or the best deal I could get was $120 per month if I used my Netgear modem. Well, I took the gateway because the rep said the new gateway would not cause any interference because they were pre programmed to block interference. Once I got the new gateway, I'm back to the extremely annoying sound interference. There is no way to adjust the channels to allow me to switch to a less traffic channel. I have confirmed that with the Xfinity gateway, I have interference. When disabling the gateway and play sound through my sound system there is zero interference. The problem is with settings in the gateway. It seems my only option is to use my Netgear Nighthawk and pay $35 more per month to use my own modem. Can anyone suggest a work around so that I can use the Xfinity gateway saving my $35 per month?