All these results are my opinion. I had some others around me that have been in the car audio business for ~15 years and there thoughts were for the most part in synic with my own. Take that for what is worth. All boxes were built to manufactory specs and built with the same style of bracing. The amp used for the test was one half of the Phoenix Gold MS1000ta, its specs are...
250x2 @ 4ohms
500x2 @ 2ohms
1050x1 @ 4ohms
Car tested in Dodge Intrepid 2002. All subs were crossed at 80hz with a 30db slope off my alpine pxa h701. Speakers were dyn mw 160 and dyn md 100.

IDMAX 12d4 v.3
1.5 cu. ft sealed
Retail: $450
run @ 8ohms

What can I say, I have read all this hype about the sub through the forums over and over. looking at the sub lived up to it, it's build quality on this sub is amazing, and its looks o so ****. Once I got it powered up it met all of that hype. The subwoofer was the smoothest sounding next to my old IDQ 15", but had a little more upmh to it. The bass dropped amazingly low, but yet recreated the high range 60hz-80hz incredibly well, and got incredibly loud off of only 500 watts. It is the type of woofer when you want can play seamlessly with your midbass and then when you get in those moods make your chest thump. Build quality on this sub is amazing, and its looks o so ****.

JL W6v2 12"
1.4 cu ft. sealed
Retail: $400
run @ 8ohms

W6v2, another well hyped sub that I had heard in a few "not so great" install before, but I have to say when installed properly this sub impressed me. It played tight and blended with my mids great, the bass sounded as if it was upfront. It also dropped pretty low, but not with the authority of the IDMAX, FLATLINE, or RSd. Also didn’t quite have the output of the three other woofers I mentioned. But as a pure SQ woofer this would be at the top of my list.

Arc Audio Flatline 12D2
1.25 cu ft sealed
Retail: $500
run @ 4ohms

This sub to be honest disappointed me. It was the loudest sub out of the bunch most likely because it was run at 4 ohms obviously, but I felt it was lacking quite a bit. I feel that it wasn’t happy off the 1000 watts and that it wanted more, and the 50hz-80hz on this woofer was terrible, it was sounded like a sub and did not blend well with the mids. It was boomy and missed many notes that the other woofers were hitting. Under 50hz its response was about the same as the IDMAX, but the IDMAX at half the power sounded a little better and almost got as loud. My impression of this sub is an inefficient version of the IDMAX.

Alumapro Alcemey RX sq version 12D4 (newest version)
1.4 cu ft sealed
run @ 8ohms

This sub was by far the best in the 60hz-80hz range. Sub was awesome up there, I previously said the other woofer blended seamlessly with the mids, well after this sub the others didnt compare. But 45hz on down I was disappointed with this sub, the output was very weak compared to the other woofers. IMO this sub reminded me a lot of Boston speakers or seas speakers, very neutral, and I think could make some on into that "neutral" sound would be extremely happy with this sub.

Phoenix Gold RSd12d
1.8 cu ft sealed
run @ 8ohms

This sub, I had hanging around, from PG and figured why not add it in. It was the prototype model that went into production, thus no magnet boot and dust cap not labeled. Well when you look at this sub verus the other you almost laugh I put it into the test, and I was thinking the same thing. Any how I powered it up, and I WAS SHOCKED, to simply put it. This woofer was amazing, it output was neck and neck with the IDMAX, it was very tight in the 50hz-80hz range, and hit low with authority. The sub is in the same league as the other and was better then most all-round actually.

Since the subs were listened to all in one day and changed constantly throughout two weeks I think I can do a fair number rating. I will also add RF p1 12"(worst woofer I ever had), and JL w3v2 12" for people to have an idea where these subs lay.

On scales 1-10(1 as the lowest, 10 the highest)
Alumapro RX=10 (tight, extremely tight, and played every note)
IDMAX=8 (hit every note with uphm but not as tight pro and rsd)
RSd=8.5 (hit the notes very tight not as tight as the pro, but had a ti bit more uphm with the notes)
Flatline=4(Was not great, just your avg woofer)
w6v2=8(very similar response to IDMAX)
w3v2=4 (your avg woofer and to me little boomy)
p1=1.5 (p1 enough said)

Alumapro RX=5.5 (hit the notes just didnt have the outpute of the other woofer here)
IDMAX=9.5 (great down here hit all the notes, accurate, and hit em hard)
RSd=9.25 (right there with the MAX but was just barley a little behind)
Flatline=9(great down here but not quite as accurate as the max and rsd)
w6v2=7.5(accurate, but didnt have the output of the rsd max and flatline)
w3v2=5(decent down here but not in the league of the other woofers)
p1=1 (look above)

The winner of the bunch with no price involved would be the IDMAX and the RSd. When you look at price the RSd is hands down the winner. The max wins in build quality. The only difference in the woofers sonically is the RSd hits a little tighter and more neutral where as the IDMAX has a more of a robust sound. Which one would I choose many will ask well, the RSd it is less the 1/2 the cost of the MAX, and performs just as well so id go that route, but if prestige is your thing then go with the max.

fyi I ordered 2 RSd at the end of this test.