One thing that can have an unbelievably strong effect is also wall proximity, both behind the amp and in front. Bass waves have such a long wavelength that they literally aren't audible in spaces smaller than the frequency demands. Also guitar amps, particularly open-backed Fenders etc. can sound drastically different when placed against a wall/in a corner; especially crunchy sounds can change totally. Battled with these issues a lot gigging in very small venues back in the day, both as a bassist and guitarist.