Studio Sub

This would most likely depend on the the size of your listening space, the style of music you are trying to create and  - what kind of acoustic treatment you have or plan to install. Most importantly - Are you using your monitors for critical mixing mastering type of work? or are you more into having a kick ass jam room ? Subs are great for that. They will only fill out the spectrum in the 30 hz to 40 hz zone when set-up for critical monitoring and when used with decent sized / quality monitors. I generally roll off at about 40 hz for the adult contemp / R&B / jazz stuff I do.  I happen to have a KRK 10S sub sitting in a box unused. PM if interested.

I now have 3 subs I picked up at Goodwill all for under $30 each.
2 are Yamaha's and one is a JBL. 
They all go Boom , Boom 
I took the bigger Yamaha to a gig once and had to turn it down a bit. Was a small room but still I didn't expect it to push out that much sub bass. I used to drag around a 18" powered sub. Won't fit in the Subaru.