For that price range you can (almost) build the best DIY system that exists; or certainly a very-very good one.
There are pre-cut kits that you can buy and glue together, or if you have your own router and tablesaw you can go commando.
If you are interested you can re-ask this question in this forum area and they will guide you:
Generally a $600 SI-18 DIY sub (aka MartySub) will beat a Velodyne DD-18 or an SVS-13 Ultra in both sheer-output and SQ.
But usually the box doesn't look as nice, especially if it's your first build. (Are you about looks or performance?)
The SI-18 is considered a good entry-level DIY sub, and because of it's price and performance it is the most popular go-to sure-fire option for most people. Especially for larger rooms that need higher SPL (those at or above 3000cuft.)
For speakers, 3 $390 Fusion-Sentinel-15's beat my 3 B&W Nautilus 803's in a direct comparison for movies and music. (Me having owned B&W 803's for 6 years until that point.)
The Sentinels can be ran off on any receiver because they are 8-ohms and 100db/watt, unlike my 803's which are 90db/watt and dip down to 2 or 3 ohms and thirst for big external amps.
They go Louder (up to 120db), More Dynamic, have More Bass, less heat, less power draw. Pretty much everything an action movie demands.
They also have smaller bookshelf models for matching surround speakers.
This website shows various Commercial and DIY subs compared to each other, and their costs:
This here is a fairly popular thread for comparing AVS-Members system's to each other, you can download the spreadsheet and take a look at what the top rigs are running with these days and what their output is:
There is lots of good and existing thread info on AVS to read into.
Pretty much at this point: all possible questions that can be asked and answered, already have been... AVS has pretty much indexed the whole AV-universe already.