Myth: Different sized subs should not be used in a system because it causes cancellation.
Misconception. The fact is that adding area will increase sonic output. It doesn't matter if you run 10's and an 18" together, you will be adding output. But, and here's the caveat, if you run ported subs the enclosures MUST be timed the same and the phase must be as close as possible. Improper application will result in phase cancellation owing to differences in box design and wildly differing group delay plots will make the bass sound anywhere from merely odd to downright bizarre. On the other hand, should you run sealed, you don't have these issues and will notice no problems as long as you're not running all the subs in a common chamber.
Myth: EQ is evil!
Misconception. EQ is one of the most abused and misunderstood tools in the audio arsenal. EQ is supposed to be used to address a less than optimal listening environment by bringing the response of the system as a whole to as close to flat as possible. Often times it's misused by the uninformed to give their system a boost to their ideal sound. Rather than address what is lacking (most often power or quality of the speakers) they use the EQ to give an artificial power boost at their favored frequencies. For instance: Not enough midbass? Crank the 60, 80 and 110hz sliders and it sounds fuller but now your mids are popping. The proper fix would be either bigger midbass, more power or better enclosure for the mids or possibly all three. In this case the EQ is being used as a bandaid. Same goes for any cases where there's a lot of EQ being applied. Boosting mids, treble, bass, low bass... you're doing it because there's a lack of power in those areas not because it needs a little tweak. Ideally EQ only needs to be used to a tiny degree, if you're going over 1 or 2 db boost you need more power, plain and simple.
Myth: Distortion ratings are the end-all be-all to an amp's sound.
Misconception. While distortion numbers are a good indicator of an amp's capability it's not the defining character to an amp's "sound" An amp's sound should be "none at all". Amplifiers are straight wires with gain. They should add nothing to the signal. Any coloration is worse than a few tenths of a percent distortion. Less than one percent is indiscernible to the human ear. Speakers will cause far more distortion to music than an amp ever will unless the amp is being overdriven into hard clipping. What is important is the s/n ratio and dynamic headroom ratings. +3db dynamic headroom is the ability to double it's electrical output. This is a good thing as it means that momentary peaks will have authority. Likewise s/n ratio tells the amp's ability to reproduce music above background noises like alternator whine. The larger the number, the better. I'd much rather have an amp with a s/n ratio of -115db and 2db of dynamic headroom with a THD of .9% than an amp that has a s/n ratio of -85db and no dynamic headroom or no rating and a THD of .02%.
That's all for now but I'll add more as I can think of them... as long as my thread remains unlocked.