You have two goals that don't match....10 Hz, and $300, you can't really get both.
Having said that, under $500, I would look at the offerings from HSU Research. At $500, a top pick is the SVS PB or SB1000, depending on room size and if you want it to work well for movies along with music. PB1000 will have much higher output around 20Hz, SB will extend lower in a small room but have substantially less output around 20Hz.
By the way, there is very little content in most music below 40 Hz.
You have two goals that don't match....10 Hz, and $300, you can't really get both.
Having said that, under $500, I would look at the offerings from HSU Research. At $500, a top pick is the SVS PB or SB1000, depending on room size and if you want it to work well for movies along with music. PB1000 will have much higher output around 20Hz, SB will extend lower in a small room but have substantially less output around 20Hz.
By the way, there is very little content in most music below 40 Hz.