For subs I'm down to two choices. Opinions? I like the Polk for the extra wattage, but the JL will save me some money and I know JL is quality stuff.
Polk Audio MM840 Mobile Monitor Series 8" subwoofer at Crutchfield Signature
JL Audio 8W1v2-4 W1v2 Series 8" 4-ohm subwoofer at Crutchfield.com
Either would be powered with an external amp at the proper power rating.
Sorry, for taking a while to get back.
If the specs are accurate, the Polk is the better choice by a country mile, IMO.
Output from a driver is simply a function of displacement. The Polk is rated at a hair over 2.5x the displacement of that particular JL. I'm sure the JL still has considerably more output capability than the stock driver, but for relatively similar money I'd pick the Polk based on the specs alone. I also took the time to model the Polk in a 17 liter sealed enclosure and it looks like it would work very nicely.
I assume by "extra wattage" you're referring to the power rating. That number is virtually meaningless. At best, it's a thermal rating of how much power the voice coil can dissipate without melting...there's a time component to be considered as well, but the manufacturer usually don't specify how the rating was determined (there are established engineering body standards that could be used). It also doesn't mean you "need" that much power, even if it's an accurate thermal rating. Almost always, a driver is power limited by excursion before it's thermally limited. The sim I ran of the Polk said it could handle 300 watts without hitting Xmax in the JBL's 17l enclosure. Doubling power only gets you 3dB increase in output so anywhere from 150 to 300 watts would be fine. Don't forget you were likely using less than 30 watt peaks with the stock JBL sub, based on the amp's clean delivery rating.
The other factor in a driver is sensitivity. This is how much displacement you get from a given amount of power. While there is some variation from driver to driver, it's not likely to be huge. The Polk appears to really be about 82dB/1w/1m sensitive if run out in the middle of a field, not inside a car. Think about how loud even 82dB is for a minute...a typical push lawn mower is in the 90dB range. This is what I meant about not "needing" the driver's rated power. If 1 watt will get you 90dB in the car (cabin gain over that empty field), think about what even a little 100 watt amp would do. Add +3dB for every doubling of power...1,2,4,8,etc.
I actually like the look of that Polk a lot, thanks for making me aware of it. I do wish I could find some 3rd party testing that confirmed its specs. Crutchfield was showing a couple of Blaupunkt sub amps a couple of days ago for less than $80 that would make a tasty combo with it. One of these days I may actually justify this upgrade for myself.
Let us know how it turns out either way...and take lots of pictures.
-Brent
