I would like to be pointed into the right direction for an accurate hitting sub. I want something around 300 dollars. And do you think the subs should stay around 500 rms so its not just some head beater bass. I am thinking about a 4th order ported box. I have a Boston Acoustic 10" in a .7 cubic ft sealed box. I am pushing 300 rms with an Alpine M500.  Do you think I should just make a new box for better sound or just replace everything?

Is the budget for amp, sub & box?

So you want a 300 dollar sub that's 500 rms in a fourth order that's "accurate hitting"? I'm not trying to be an ass, but you need to know what you actually want so we can help you.

The power handling rating of the sub has absolutely nothing to do with how "accurate" the sub is.

I'd look for a sq driver like the Fi Q. Sounds great and in the right boxes can get loud

How much room do you have and what is this going in?

I'd look for a sq driver like the Fi Q. Sounds great and in the right boxes can get loud

He doesn't want an SQ driver, not even close.

The budget is for the sub. I will work around the sub for the rest of the budget.

Ethos!

 I want sound quality  over loudness, but I still want loud. LOL 

Depending on which Boston Acoustics sub you have, I would build a ported enclosure for it. If that doesn't satisfy your needs then I would consider more cone area and double the amplification power.