Best used sub for $300 and $600?

It really depends on your preference for music or HT.
For around 600, a good combo sub for both is the Velodyne
f1200x or the spl 800. The Velodyne ULD series although older are great subs and very musical if you just want to add a sub for 2 channel. Again, I am partial to Velodyne due to the price/performance... Others to consider are
Sunfire... gets very nice reviews and possibly REL.
Best of luck,
Joe
Custom Audio LLC

Thank you Joe. Music is my preference.

HSU VTF-2 in the $300 range...
REL Q100E or Stratus 2 in the $600 range..

I would look into a new SVS PB1-ISD for $599. I just replaced a REL Q201e with one and it blows the REL away IMO.

www.svsubwoofers.com

Take 500.00 go to Acoustic-visions.com , order an Adire Tumult.I now own two.You only need one.You have NEVER heard or felt anything like em.I have owned them all.This was suppose to be a hobby sub to have fun with.I never thought it would surpass the big 18" servo's.Good luck

IMHO,

Under $300 - Paradigm PDR-10
Under $600 - REL Q's or HSU VTF-2 ($500 new)

Under $600 REL's (lower end Q series or older strata's) are very rare. But if you can find one, definitely get it.