I have a Spectrum ES2251 Modem, with my XR1000 connected to it. My PC is wired to the XR1000, and I use a few other devices over WIFI, all of which are connected. I have never messed with modem settings.
I pay for 600/20. Download is usually a little higher (say 630), and upload is in that range. I have manually entered and alsoused NETDUMA tools to detect these speeds.
I have played around with congestion control but it doesnt seem to have solved the issue.