What crown?
Kinda vague question... with many conditional answers.
There are many 'loud' subwoofers in that price range, but are you just interested in something that 'sounds' loud for $300, or something that does numbers on the meters?
What is it your really looking for here?
From a strictly competition angle, I can tell you that in MWSPL, for 1000 watts and under per woofer, Kicker (L5/L7) pretty much dominates the top vehicles taking home the trophies (3 years running). I speak specifically of the Basic 1-4 classes. The majority of these installs are going to be 'burp' cars, meaning, they don't play music very well (mine included).
They also have a strong heritage in the dbDrag and USACi organizations.
Bottom line though, the install is going to play a much larger part in 'being loud' than the equipment itself.
If your looking for something for your daily beater though, pick what 'you' like. We can't tell you what will sound good to you.