From your description of your severe RFI on the higher bands, I think there is a very good chance that much or most of the RFI is due to actual RFI picked up by your antenna which is differential mode, so a coax (common mode) choke in that case may not help much or at all. However, that being said, a coax choke will not hurt anything, and MAY help since you MIGHT also have some common mode RFI. Some folks always like a common mode choke at the antenna feed point, others consider it an option, since it does not always help. I mentioned the coax shield connection as a possible problem, although I have not found it common in my 67 years as a ham and decades in the industry. I did have a recent problem where there was a one ohm resistance in the shield connections in a coax switch. That was enough for my coax to pick up several RFI sources inside my house as my coax snaked through my house and caused noticeable RX RFI on 160 meters. I Corrected the coax shield problem and could no longer hear this RFI since my 160 meter antennas were 30 to 250 feet from my house. I doubt that is your problem, and I just mentioned it since you need to keep an open mind when troubleshooting noise problems. Strange things happen. Be sure to record your noise readings every time you change anything since often noise comes from several sources, and you need to keep track of which changes help as you "kill" the RFI one source at a time. Happy noise hunting Rick KL7CW