im running a passive mixer into my FX loop to cut some highs from my amp. is this safe to do?
Should be fine as far as I know. But I would get an EQ at some point.
I believe Chris Taylor (bassist of Grizzly Bear) runs a mixer at the end of his effects, but I believe that's primarily to keep the levels up on all his instruments, since he runs bass, accordian, flute, clarinet, saxophone, etc. through the same fx setup and amp. If you just want some high end removed, get a simple EQ pedal like a Boss GE-7.
whats the difference between running it in front of the amp and in the FX loop? also is it allright to use an EQ as attenuater like crakning the amp and lowering the volume on the EQ in the FX loop?
No, it's fine using both the mixer and the EQ like that. Many people use a volume pedal in the FX loop for that same purpose.