I would suggest the Buttkicker Amplifier for $252 delivered from Amazon. I've had mine for a month now. Other people report good results as well.
The SS 15" RL-p15 is $279 not counting shipping.
Or, check out the Fi SSD15 or $209 delivered.
https://ssl.perfora.net/www.ficaraud...iew.shopscript
The xmax is a little less at 21 mm, but you'll hit your budget target.