Alpine type s 12 dvc 4ohm advice

I installed 2 of these in a sealed enclosure with 1.15 cu per side.  Wired to a 2ohm and being pushed by a mrd605 alpine amp in which I think is rated at 600 rms x1 @ 2ohm. It sounded ok but it was not very impressive for how much hype I have heard. For kicks I put in a sealed enclosure with 2 entry level concept hz-124's in  it was noticabley louder (yes I adjusted gains etc..). I don't have a lot of experience with alpine newer stuff so I am looking for advice, maybe the subs perform better in a ported enclosure, bigger sealed enclosure. I am looking for ideas before my customers wants to me to build another box for 3 type s's.

I had a fellow installer that sells/installs alpine drive up and take look to see if I was missing the boat. We changed the volume in the box by adding wood blocks, we then tried stuffing it with poly fill nothing seemed to work.

Pardon me if I'm way off base here, but how did you manage to wire a pair of 4ohm DVC subs down to 2 ohms?  Are you only using one coil on each sub?  If so that might explain your lack of output.

I run a single type S at 2ohms giving it around 350W RMS with a Pioneer GM-7100 in a 1 cf sealed box and I think it sounds pretty good for a fairly low priced sub.  Just my thoughts though.

I checked my notes on the install I had them dropped to a 4 ohm load (my bad)so they are only seeing about 400 watts. I will get them to let me build a bigger box, its in a taurus so plenty of room to experiment. I called the customer this morning and he emailed the seller on them (fleabay purchase) they were used but only for a couple of months, sure. So now I am wondering if they are just played out. Either way Its not hard to pump out a bigger box for them.

Should we try a little more power to them, thats my customers stand point, he feels they are under powered. He is contemplating running 1 605 per sub. Any thoughts...