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if your speakers are large standmounts,and you centre speaker is a large speaker to, i would set your speakers to large,if you don'nt the speakers bass will be took away some what and your speakers will be left sounding empty, and it will send it to the sub - the problem with that is your be distracted by the bass from the sub quite often - your speakers and overall sound will not sound full bodied,set amp crossover to 80hz like you have, set your sub crossover half way 12'0 clock or just under,another way of setting the correct crossover is play some bassy music and bring the sub crossover up until it intergrates into your main speakers bass frequency,of course your need to have your main speakers set o large