Your not crazy.

Your sub does a better job in the lower bass category, even if your mains frequency response is capable of going lower. A 10-12" woofer will push more air than a smaller 6" woofer. Also, a sub has a built in amp, so your receiver is not required to work as hard.

I have my m80's set to small with an 80hz crossover matched with an ep600sub.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85