The Kenwood KAC-X811D will supply 500 watts RMS into a 2ohm load.
An appropriate sub would need to have an RMS rating close to 500 watts RMS, so around 400 to 600 watts RMS. The sub will also need to have a nominal impedance of 2ohms, a dual 4 ohm voice coil sub can be wired in parallel to present the 2ohm load.
If you want to run a pair of subs then you would look for subs with an RMS rating of around 250 watts RMS each with each sub having a 4ohm voice coil so the pair can be wired in parallel for the final 2ohm load.
The amplifier is rated to handle a 1ohm load but some X811D owners have reported overheating issues when using them this way. It will however stay dead reliable at 2ohms.
How much do you want to spend on the sub woofer?.
How many sub woofers do you want to use?.
What size of sub woofer?.