to improve the tightness try reversing the polarity of the fronts or sub, this might make them go inphase with each other. If there is no different, then they speakers are 90degrees out of phase.

By using a 400uF 100volt capacitor on the front speakers this will shift the fronts phasing 90degrees, effectively you will now notive a difference when you swap the polarity, one will be have tighter bass.

Another big thing is that the JL subs dont suit ported box's that much, so i would change to a sealed box if its ported. If the ported box is tuned high-ish (35hz) then the group delay would be high around that frequency. Reducing the tuning frequency to 25hz will reduce the lag on the sub.

Firing backwards or rear loading in a sedan can often cause alot of panels to vibrate liek the boot lid. These panels continue to vibrate after each bass beat, makign the bass sound loose like it is lagging. Deadening your boot should improve this also.

Firing forward will improve the tightness and get rid of the delay.

There are more expensive options such as buying a headunit with time delay but i would definitely try the above before you start spending lot sof money on new equipment.

Personally i would go with a small monoblock as your next upgrade, a 1ohm stable 400wrms coustic amp would blow the jaycar amp away and dramatically increase dynamics, volume and SQ in general

yeah, altering the box may be the simplest and most (cost) effective method. what's the current box design?

Sorry guys, I should have mentioned the box before

It's sealed, about 40L (hard to remember - I made it a fair while ago) and it fires into the cabin.

It's hard to pick whether the beat of the sub is lagging or the bass resonating in the panels of the car make it seem like it is lagging. I think it is a little of both - one more than the other in some songs.

the box seems ideal, and firing directly into the cabin will reduce any apparent lag.

is the front/baffle of the box 'sealed' up against the back of the seat? or does subbass soundwaves travel around the sides around the back into the rest of the boot too? it would be worth building some contoured mdf side-baffles to seal the box right up to the seat.

otherwise, i think you would get a significant improvement in subbass quality with a good monobloc subamp. however, it would be fairly costly in your situation because your 'D2' sub can only be configured for 4ohm or 1ohm. and entry-level monoblocs produce relatively poor power at 4ohm, and 1ohm stable monoblocs are expensive. so either way, you'd probaby need to spend about $1K for a suitable amp.

not sure there's anything else i can recommend here. you could try facing the sub in a different direction, say sideways, and see if that helps.

good luck

Thanks for all of the help guys, I'll just put up with it 'cause I don't want to spend any more money on my current install (I MAY be getting another car in a few months so I will have to redo the install anyway). Although, a good subamp would be something I could always use.... I think $1k is a little out of my price range, any other suggestions on a suitable amp? Maybe something around $500-700?

How much is that 1ohm stable coustic amp that Maz mentioned a few posts ago worth?

i would recommend it except it's in fact not 1ohm stable. it's only 2ohm stable. otherwise i'd suggest looking at it as it's excellent value around $600.