Hi Justin - the PS-1000 retails for $500 new, and usually goes for around $200 used, so that's definately a good price if the sub is in good condition!
This should definately be a nice upgrade from the JBL.
The value of electronics especially speakers drops dramatically after a few years. I had a PS-1000 for about 3 months that I had purchased from a pawnshop for $100. It was the first version of the sub in a series of 3 (they modify the sub to keep up with the times). It was good but I upgraded and sold it to a friend. It is a solid sub, but a little boomy due to the bandpass design. To reduce the boominess I packed the two of the three reflex ports (the two on top I left the one on the bottom unplugged). The result will increase the overall depth and accuracy. The pitfall of doing this is decreased efficiency, it still goes plenty load enough though.
Well, the PS line aren't that great beign bandpass to start with, so any product that is less than stellar used will drop its value very quick. I would say it is an upgrade and a decent deal, but it isn't a spectacular sub either. Maybe if you use both...
What other system components do you have: speakers, receiver ecs.....
How big is your listening area.
I forget to mention this before, but due to the PS-1000's size and Phase Control it is notoriously difficult to place so you will need to be very patient with it. There is a chance that it will not Wow! you when you first plug it in. It took me some time to integrate it into my system.
One of the ways you can set this thing up is to place the subwoofer in the listening position and play something with a lot of bass or some bass test CD, while it is playing you get on you hands and knees and wander around the room untill you find the place were you can hear the greatest output and put the sub in that location. This may not always work due to maybe that position cannot be occupied by the sub for maybe cosmetic reasons or something else.
I was using this sub in a 16x12x8 room and had a very good results. The only reason I upgraded was that I had gotten a sunfire true sub which deffinitely has some advantages over the paradigm. I would say that it is a good deal you should notice an improvement. Are you purchasing it from a private buyer?
If you want to keep the JBL you can daisy-chain the subs. This means you use the low-level outputs on the rear of the PS-1000 to connect it to the other sub. In an ideal setup you would have to buy another sub cable (75 ohms). You could also buy a y-adapter for the sub out on the receiver essentially giving you a second output.
I made an attempt to run two subs at once, sunfire true sub and an M&K, with bad results. The bass was very limited.