I know this might sound out of order. But third time in a row is simple negligence and miss-care with settings heavens above I don't know how you could have possibly done this three times in a row!
I've been running the JBL 4645 cinema sub bass with a newer JBL 2240H sub bass driver now for the past 8 years and not a single days trouble with. It might have something to do with that fact that I've worked in the cinema industry for UCI and Warner Bros as projectionist some take it quit literal and some don't.
Mate I may have solution a very cheap and affordable way of keeping an eye on the exclusive baby boom track or as it is called now LFE.1
The room hasn't really got anything to do with the size. The Empire Leicester square in London used to use up to x8 JBL 4645 and not one of them ever blew! The Empire has been refitted this summertime 2006, with THX and JBL with a whooping x16 JBL 4645C subs behind the screen, which is 13 feet wide and 8 feet high!
Now the reason why you're probably destroying the JBL 4645H is! Overdriven with too much input going into the amplifier, I bet you £1.00 pound UK! That you're LFE.1 content is exceeding 0db!!!
Here's my JBL set-up note the JBL 4645 placed to the left-hand corner, you can't miss it it's LOL larger than the flipping SONY monitor!
The AVR set-up is somewhat complicated and yet it so simple with a few special modification, note the stereo audio mixer above!
All outputs from the Kenwood KRF-X9050D THX select uses RCA phone outputs this gives the user greater flexibility!
Also my set-up goes though a rigorous special processing where I can control the mix within the Dolby dts 5.1 domain!
Now, sub bass extension and LFE.1 content which is set to 0db with THX bass manager tuned to ON! Helps keep the LFE.1 from exceeding 0db when the Kenwood KRF-X9050D THX fader is set to 0db!
Also the outputs for left and right monaural sub bass extension passes into the audio mixer where it is balanced, centre channel monaural sub bass extension also passes into the audio mixer where it is balanced surrounds are also balanced and all this via separate inputs!