WoofersEtc.com - IDQ10V3.D4 - Image Dynamics 10" IDQ Version 3 Subwoofer
I know this is a bit expensive especially since you have to have two for your current enclosure but I have heard really good things about image dynamics, this way you would have new subs and not have to take a chance on buying used again and risk having issues. (I've also heard memphis subs are nice as well)
Another option would be to go to a single JL10W6 or a JL12W6, combined with that JL 500/1 would give you all the bass you need with your cars cabin space and be less weight and give you more trunk space.
If your going used, take a multimeter, check ohms, if it's something crazy high then the voice coils are usually gone or damaged. Check the surround really good and for sure plug it into your car and listen to it and watch the action on the sub itself.
Gain setting on the JL 500/1. I have a pac LC1 controller mounted right under the head unit to adjust the sub level up and down. I set the dial dead center on the LC1. (head unit sub vol all the way up) Then choose a few bass heavy Dub Step or Trance tracks, set the gain on the amp to start with at 1/2. Get in the car and turn the stereo up and listen to several different tracks. If the level is too low then I bump the gain up just abit and test/listen again and repeat, if the bass starts to get loose, boomy or sloppy then I know I have I need to back the gain down.
On my JL 10W6 I have the Pac LC1 set at center vol, then on the amp the gain is in between 1/2 and 3/4 (around 2/3rd). This works out perfect for me as it's clean, clear and tight and no boomy loose or distorted bass at high volume. If I come across tracks where I need to adjust I can go less or more accordingly from the center point on the pac lc1 controllers dial.
I'm not worried about having my gain set on the amp to it's optimal max output level but more to where the bass matches the cabin space and existing system so it does not overpower the main speakers and blends nice and gives me adjustability on the dash without having to go into the menu on the head unit to adjust the sub. It's definitely better to have the gain below the possible max setting rather than risk distortion and damage to the sub itself.
Had the 10W6 in the ported box not been enough bass for my system (hertz audio speakers and JL 300/4 amp 75 watts per speaker) then I would have looked at using 2 subs. I had a 10W3 in the car previously and it sounded great but I felt like I was pushing it's limits. The 10W6 is definitely louder, much more accurate and cleaner at high vol.
BTW a good carpeted single ported enclosure shouldn't cost you more than 50-60 bucks, check around with your local shops. I paid 55 for mine and then added corner protectors from
crutchfield...