Replacement speakers

David Morison

Aug 21, 2012
Aberdeen, Scotland
Will be easier for anyone to give relevant advice if you can give more info about the cabs themselves.
Assuming they're vented, the net internal volume after accounting for drivers, bracing etc and the cross sectional area of the vent(s) & vent length would really be needed.

Paul OBrien

Apr 8, 2014
What are the internal dimensions of the boxes and what size and how many ports are there. If it turns out they are tuned to 35-40hz like most then there are many drivers that could be used and all of them are higher performance than the old JBLs, but it's actually possiblle to go too far with the drivers if the ports are small and cannot be modified.

Paul Lewendon

Oct 8, 2019
British Columbia Canada
If the subs are blown, look into a re-coning kit. It's more intimidating than hard if you're a handy person.
It was very intimidating the first time I did it, and I have done many, but if you follow the directions, they'll be as good
as new and you'll save a bunch on money. And as an added bonus, a great sense of accomplishment and good feeling
knowing they didn't end up in the landfill.
It's better to find a drop-in kit as opposed to the kit that you have to assemble everything in the kit
including the voice coil to the cone.
All kits should be drop-in kits in my opinion, less to go wrong, and alot quicker, and cleaner looking.
Factory ones are a little bit more expensive, but if you buy a good quality aftermarket one from an established dealer,
they're just as good... well maybe not absolutely 100%, but the majority of people probably couldn't tell the difference,
if it's a substitute or not, again, if it's a good quality one.

Perry Wright

New member
Jan 17, 2019
Waverly Iowa
Without a doubt, Eminence SIGMA. Will probably sound better than the originals. Best cost to performance ratio and with sensitivity of 99dB, they kill the competition. I would not even consider anything else.


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Even with those JBL 151's in perfect condition you will never get close to modern double 18 subwoofer preformance from
them and you will end up with more blown 151's if you try.

Paul brought up some good points, let us know the cabinet information.
Not even looking at the super monster 18's that are available today there are many mid level 18's that would probably drop right into those cabinets and out preform the 151's that only had a 300 watt power rating with something like a 4mm Xmax.
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Art Welter

Jan 11, 2011
The 150 watt JBL K151 only has 2.54mm Xmax- even the relatively lame Eminence SIGMA18A-2 has a 650 watt rating with 6.1mm Xmax.
Doubling Xmax (linear excursion) can allow for 6dB more low frequency output. There are now 18" drivers with as much as 10 times the Xmax of a JBL K151, though they would cost more than $500 per unit.

Anyway, Randy (the OP) has not replied in over 3 weeks, so not much more to discuss..

David Morison

Aug 21, 2012
Aberdeen, Scotland
Any modern driver will have enough extra output potential compared to the 151's that 2 4" ports will be nowhere near enough.
How much woodwork are you willing to do? If you're not prepared to at least cut holes and install a few extra port tubes, there may not be much improvement to be had, IMO.
It may also be worth increasing the amount of bracing (depending on what's in there at the moment of course) if you want to up-gun it much too.
Even the Eminence Sigma Pro mentioned up-thread (not a particularly high spec driver nowadays TBH) would merit at least 4 4" vents, if you were planning on being conservative and never taking them past their nominal Xmax, which would occur at well under full program power for those drivers.
As David said for any modern'ish 18 two 4 inch ports are not going to work.

A quick run of the numbers figuring your box with 10 cubic feet of internal volume the two four inch ports
will just work with the 151's, that's a 40hz tuning and a port length of 1 1/4 inches.

With an Eminence Sigma or bumping up to a Definimax they want at least four four inch ports
about 5 1/2 inches long.