As flavoie mentioned, the strong signals being amplified by your CM preamp may be overloading your tuner, but I suspect it may in fact be the CM preamp itself that is being overloaded first. I've probably been looking in all the wrong places here (i.e., search terms...), but I haven't seen much mention of this.That reminds me...for some reason if I point my antenna right at camp fortune I get lots of MB on CBC (unwatchable really)...when I point it 180 degrees the other way with the signal coming in the back it seems just right. I havn't tried it without the preamp though. I am assuming the bowtie side is the "front" of the antenna and the jail bars are a back reflector(?).
There is a spreadsheet posted online (not sure if rules permit linking) by holl_ands that analyses preamp overload characteristics, and some are not suitable for handling very (very) strong signals (e.g., CM 7777).
This is something I've been trying to wrap my head around lately, and have been planning to put together a description of my situation for input by the gurus here.
So, aiming away from Camp Fortune may be required when you have the preamp inline.