I just wanted to add something here. I do encounter this problem sometimes and it is not an ingress issue caused by coax and passive impairments. Not saying I don't encounter ingress issues. These are the first channels to go if you have incorrect input/output levels that cause CTB/CSO/XMOD issues. Long cascades beyond 6 actives will cause this also when using amps not designed to support longer cascades. The priciples of thermal noise in a system gets closer to the carriers much faster in the lower frequencies. This is when CSO/CTB distortion come into play, much as they would in the longer cascades and not inputing at the designed noise figures or overdriving your hybrids.
This is CABL.com posting #363956. Tiny Link: cabl.co/mbGQq