High Current Amp ?

He was talking about using three of them, that's where I got 1.33 ohms.  But I agree with you that he's better off facing all of the subs (whether he ends up using three or four) in the same direction.  There's been a lively discussion on rec.audio.car about the effects of rear-facing versus front-facing subs, including some SPL graphs that show a pretty significant difference.  So to avoid unnecessary cancellation, I'd try to a) locate them as close together as you can, and b) face them all approximately the same way.

Scott Gardner

Did you decide on a ported or sealed box for the four subs?  If you go with a sealed box, you'll need between 2.0 and 3.6 cubic feet of airspace, AFTER you've subtracted the volume of the subs, bracing, etcetera.  If you go with a ported enclosure, the requirement jumps up to 3.6-6.0 cubic feet.  That's getting to be a big box for the back of a 3-series Bimmer. 

Have you measured the available space yet?  If the customer wants 6-8 inches free on top of the box, you can't put the four subs in a 2X2 array - it'll probably be too tall.  You probably can't line them up in a row, either, because the subs themselves will occupy 44" of width.  If you chamber the box to give each sub its own airspace, that's three dividers, plus the left and right wall, for another 4" or so.  Unless you can stagger them or come up with another creative arrangement, you may be stuck trying to put 10 pounds of dirt in a 5-pound bag.

Scott Gardner

Well, I'll start off with the amp.  Looks like a good amp, but 600 watts into the 2-ohm overall load you'll get from two of the 10W0 subs is overkill.  Never hurts to have too much power, though - he may want to upgrade subs in the future and the amp will be ready for him.

I wish I knew what your customer meant by "wanting it to hit hard".  Putting 250 Watts RMS into a pair of 10W0s, he may be pleasantly surprised.  In my old Jetta, I was putting about 375 Watts into a pair of JL 12W0s, and it was plenty loud for me, even in a sealed box. My wife's car has a single 12W0 in a large sealed box, being driven by 100 Watts RMS, and it's loud enough for her.  Granted, her Toyota Paseo is a good deal smaller than a BMW -series, but she has half the speakers and less than half the Wattage that we're talking about with the Bimmer.

I'd try to talk the customer into just using two of the subs, and building a nice ported box to get the extra SPL.  You'll have to be careful with the gain on the amp, since the ported box won't protect the subs as well as a sealed box will.

Scott Gardner

The PG amp you linked to earlier still sounds fine, or he could go with something larger now that he's considering the W6 line (See below).  Whichever way he ends up going, either with the W0 or the W6, he'd probably be happier with a ported box, if he wants serious SPL levels out of two or three tens.  The 10W6v2 would really be the best bet, since they have so much more power handling (400 WRMS) compared to the W0 (125WRMS).  Here's a link to the data sheet for the W6 subs:

http://www.jlaudio.com/subwoofers/pdfs/W6v2_MAN.pdf

The only drawback to the 10W6 is that it's got dual 4-ohm voice coils, so with two of the subs, your only real choices are 1-ohm overall or 4-ohm overall impedance.  The PG amp we're talking about makes max power into 2 ohms, which wouldn't do him any good in this case.   Better to look for something that's 1-ohm stable.  Here's a possibility from JBL:

http://tinyurl.com/2ydoo

It's 1200W into one ohm.  That's more than a pair of even the W6's can handle, but it will give him some headroom and more power if he wants to upgrade later.

Good luck,

Scott Gardner