My Mark Levinson sub has been in for over 2 years now and it still sounds excellent. It actually sounds better with age because the woofer seems to have broken in and allows for more reflex. I am completely satisfied with it and would recommend replacing the stock Mark Levinson sub (which is pure junk) once it blows. The JL sub is 100 times more sturdy than the Mark Levinson sub. The sound quality and loudness of the bass is noticeble even without an aftermarket amplifier. I had the Mark Levinson sub on my 2GS replaced with a JL Audio 10IB4 sub. It is 8 ohms and is a free air sub. It was a direct drop in the stock location replacement with no aftermarket amp needed and it sounds great. A few others have done this and have been very happy with the results. You can only use the 8 ohm sub to work with the Mark Levinson amp. They are very rare subs.
Unfortunately, the JL Audio 10IB4 sub was discontinued. But it was replaced by the JL 10w1v2 8 ohm! Here is link to where you can buy the sub: