Coax amplifier issue

I've had some issues with slow upload speeds on my 2-gig plan, with downloads being fine at around 1900mbps but uploads only at 50 of the advertised 250mbps. After troubleshooting for a while, I think I've found a solution. In my network cabinet there is a coax amplifier rated for "5-42mhz" between the cable from the street to my modem, which is supposedly meant for sub-split speeds. Removing this and directly connecting Xfinity's XB7 modem to the street coax increases upload speeds to ~200mbps, but causes some other odd issues. I believe replacing it with an amplifier rated for "82mhz" mid-split speeds (whatever that means) will fix my issues. The only thing is, I can't find any for sale. Do I need an Xfinity technician to take a look at this? Have I totally misinterpreted my issue and solution?