I am having the same exact issue! I have an SB6190 Modem (less than 3 months old) and also I have a brand new Linksys EA7200 router.
I disconnected all of my splitters and coaxial from the amplifier today. I now have 75mbps instead of 50mbps. I got on with a chat agent and he checked everything and sent me to advanced technical support. I am waiting for the phone call now.
Let's see what happens.
Do you have the same setup?
basically the same for us...upgraded to the 600Mbps from 175. So, I got a new SB6190 because my old surfboard could only handle my previous speed.
But, now the top download speed I can get is 90-100. Before switching out my old router this morning, I was getting 135.
So...not real sure what is going on. The tech didn't either. yay.