The Empire is in a whole different price class though. It is just under $1K shipped vs $350 shipped for a PA-150.
The PA-150 loses a lot of steam after 30hz, but it does the 40-80hz ranges very, very, well for the money. If you are looking for a subwoofer for gobs of output and extension for under $500 you are not going to find it (probably need to double your budget). Either spend more money or determine what sacrifices you want to make.