those signal levels look fine to me.
Arris modems are generally used for people who have phone and internet service. From what I've seen at-least.
Are you sure that you have 8 Downstream channels in your area?
SBG6580 appears it can do 8 Downstream Channels. IF you do buy a device make sure it's listed at »
mydeviceinfo.comcast.net otherwise, they wont provision it.