I have a jl 15W3D4 and I told the stereo place I wanted it wired so the amp sees a 4 ohm load--they said they did wire it that way--is this possible--I would think it's only possible if you don't use one of the voice coils--(if I'm right)what are the effects, advantages/disadvantages of doing this (using just one voice coil)

 

Can't wire it that way without a zobel. Don't ask them what a zobel is because it sounds like they are electrically challenged, like most install shops. What amp are you using? If it is an off-brand, then what impedance load can it operate at in stereo, and bridged? I can't remember the electrical and mechanical reasons but you don't want just one coil connected.

 

if the amp is 2 channels, cant you wire it so that each voice coil gets its own channel? i had a single dualvc cerwin vega and it was wired that way.

 

I have the Kenwood KAC929 amp, it's a 2ch. amp bridged to push 460 wRMS x 1--the amp is stable at 2 ohms in stereo but only stable at 4 ohms bridged. It is bridged to my sub-- So should I be concerned with how they wired it?

 

If it is bridged to the sub then the sub should be wired in series, an eight ohm load. Which is only as efficient as what kickyoazz said. Either of these two ways will work, but won't get the optimal performance from the amp. If they have it bridged and the sub wired parallel, then your amp will overheat easily and likely sound like crud. If it sounds tight, just not really loud, then they probably hooked it up correctly. If it really thumps loud but the bass lacks tightness it could be from driving a 1 ohm load, which will drop the damping factor, which allows the amp to tightly control cone motion, to 1/4 the 4 ohm level. The amp will also be less stable and reliable at a 1 ohm load, as well as have the warranty voided.