Could the "POP" Noise you heard originally be the fuse "poping" the very 1st time this happened? You stated that you also had a "burning plastic" smell as well, I am assuming this all happened in seconds of each other, so I am just going by previous expereince that the poping noise followed by the plastic smell is/was the fuse blowing (AND A SLIGHT BURNING OF THE ORIGINAL FUSE.
The only thing I am confused about, is you state you bought them off of eBay as new units, but you have had them 7 to 8 years.
Since any new fuse blows immediately when you replace it, you definately have a short "somewhere" (My guess is in the actual amplifier itself), AT THIS POINT ITS UP TO YOU ON HOW FAR YOU ARE WILLING TO GO & THE TIME YOU SPEND FROM THIS POINT ON:
You may want to take off the cover & take a GOOD LOOK around the printed CIRCUT BOARD (You may see several, the main one to look at FIRST is the main Power Circut Board & LOOK for any "BURNT" OR "HOT" Spots. It may be hidden UNDERNEATH on the backside of the circut board, so you may have to remove the screws that hold down the "Main" power circut board (it will be the one that has the main power wire that you plug into the wall for main power). Try to turn it over so you can see all of the soldered parts side & look for any slight DARKENED SPOTS. If you do locate an area that is darker in color compared to the "Regular" color of the rest of the circut board, then you may have found the component/part that has failed. It could be an IC (Integrated Circut processor, THAT has an equal number of pins on each side, MOST are rectangular in size) of which there can be a lot of on the board, you can only tell the difference of each one, as it will have printed on the topside, a series of numbers/letters & hopefully a brand name or logo this is the ONLY way of crossing these letters/numbers, to hopefully find a replacement IC.
New IC's depending on its function, could be as cheap as a dollar or two, up to several dollars. Hopefully you have a good electronics stoire in your area that can cross over the numbers you get off of the IC for a new replacement part. Radioshack, will carry only a few of the "Common" IC's, but the more functions that it has, you will have to go to a good electronics service store (THE ONES THAT ACTUALLY DO REPAIRS THERE) They will usually cross-over your numbers & sell you the part(s) you may need. It could be a simple 3 prong transistor (OF WHICH IS PROBABLY A LOT OF ON THE BOARD) The main thing to look for is anything that is darkened in color, as a little too much heat can really ruin any printed circut board &/OR part, but I have run into what I have found the so called "Bad" part, but NO WAY of tracing what it actually is, other than an IC or transistor etc, but no way of crosssing it over to a replacement part for repairs.
You may have to try & find a wiring diagram for that particular JBL Subwoofer to be able to find the part that is bad. This is the only way that I know of to try & tackle a repair of something like your subwoofer. You just need to weigh in how much TIME & EFFORT do you want to spend on it, vs. just going out & buying a replacement subwoofer. I wish I could tell you of a easy quick fix, but with any electronics, the blown fuses is your only clue as what to look for next. (Its like a puzzle that you kind of follow backwards)
If you do find a "bad" part that has no identifing numbers, you will want to try to find a wiring diagram, go to: SERVICEMANUALS.NET .............THEY HAVE SERVICE MANUALS, WHICH (you need to verify, THAT IT HAS A WIRING DIAGRAM WITH IT) SOME THEY OFFER AS A .PDF File THAT YOU DOWNLOAD, SOME ON PAPER & SOME ARE ACTUALLY ON A CD, IT JUST DEPENDS ON IF THEY HAVE IT OR NOT, ALL YOU NEED IS THE FULL MODEL NUMBER & OF COURSE THE PRODUCT NAME (JBL) & DO A SEARCH ON THEIR SITE & IT WILL TELL YOU THE AVAILABLITY OF WHAT THEY HAVE (which is a lot, like well over 30,000 MANUALS, OWNERS MANUAL (not what you want, that is the paper you get when you buy an item, like how to hook it up etc) YOU NEED A SERVICE MANUAL WITH A WIRING DIAGRAM. You can look for one now if you wish, just to see if one is available, BUT I would not buy it, unless you really need to, for identifing parts etc.
I have Sony speakers, which (Strictly my opinion)
your JBL's are a little better units, than my Sonys are, but when you get the so called "higher end" speakers, Like ours, it really comes down to personal preference of JBL's vs. Sonys. If they are in fact 7-8 years old, like my Sony's, which are now 15 years old & in use DAILY in my Home Theater Surround Sound System. These days getting the time we have gotten out of our "High End" top of the line (so to speak) is the basic lifespan on most of the high end products. As they say, they dont build them like they used too, is really true, as if they did, these top line speaker companies would be out of business. Since you have replaced the fuse only to have it immediately blow again, means a severe short somewhere, as the fuse is there to blow on purpose, so nothing else shorts out & causes more damage or worst case senario, stopping a fire from over heated item, but I am sure these are things that you already know. If your "other" system speakers (I am assuming this sub-woofer is for a Home Theater system) You can just get another compatable active subwoofer as a replacement. If I were to guess, your problem is in the amplifier itself, your problem lies more than likely on the printed main power circut board. The only way to verify this is like i described above, is doing a little investigation on your part. You will have to decide on how much time you wish to dedicate to this project, or is it something that you can do yourself? If in fact this is something you can tackle yourself & want to do it, then just follow what I was stating above for your initial investigation of where the short or bad part is actually located at.
If you decide to just replace your subwoofer, I have found & used on occasion, a great Audio/Video source, which you have probably heard of if your into hi-tech audio/video systems & parts etc. is CRUTCHFIELD.COM (800) 555-9430. THEY HAVE BEEN AROUND FOR LIKE OVER 30+ YEARS. NOT ONLY do they have great Sales & knowledgable staff, BUT THEIR TECH DEPT is one of the BEST in the industry. Before you go out & buy anything, I would call them & explain to them your issue & IF you get a good TECH Person on the phone, He/She "may" be able to tell you probably whats wrong with YOUR JBL Subwoofer.
They sell the top names in speakers, ANYTHING that is home or car audio from cheap items, all the way up to the top of the line Products, home theater, digital cameras HDTV's, like I said anything & everything audio &/OR video etc. I am sorry I couldnt be of any magic help, but IT REALLY DEPENDS HOW MUCH TIME ARE YOU WILLING TO PUT INTO THE SUBWOOFER.
.......OR TO REPLACE IT I WOULD go to them (Crutchfield), as they know their stuff when it comes to problems and see if they have anything they offer as a replacement. They even have a section on their website on nothing BUT SPEAKERS, like a bunch of articles & some videos of JUST Speakers !!
Its worth AT LEAST taking a look at anyway, THEN again you can find some killer deals on eBay as well. Again, I think your problem is something with your main power circut board, but it is really up to you to decide on trying to repair it yourself or just buying a replacement subwoofer, its really up to you at this point. I apologize for a lengthy answer, but I just wanted YOU to know what your getting into with fixing it yourself vs. buying another unit.
I Hoped I Helped you in at least a decision, of fixing vs. buying.
GOOD LUCK WHICH EITHER WAY YOU GO. IF YOU NEED ANY HELP IF YOU GO THE "FIXING IT ROUTE" FEEL FREE TO E-MAIL ME AT: email@example.com