Box Question - 2 SA nighshade 15's

[Notoriousj76]

So, not that long ago I traded around on craigslist and got ahold of 2 SA nightshade 15's with swapped motors. The box they were in was tuned about 35 hz and I barely hit 130 db on a hifonics brutus 2000w rms amp. That box was crappy in my opionion, undersized and not well built/ported, so I decided to build a new one. Now I have them in roughly 17ft3 airspace tuned @ 28 hz and still barely hitting 130 on burps and flexing things like hell.

Before this setup I had 1 HCCA 15 in 8 ft3 @ 28 hz and it actually sounded better on the same amp/wiring. My quesiton is why they just refuse to hit lows desipte what I do, I know I have little power to work with but I'm in the middle of saving for a bigger amp. Should I just retune to 32 hz like the rest of everybody out there? According to WinSD 32 hz tuning would flattedn out my respone but I'd have to tear my expensive new box apart to find out and I don't want to be disappointed if it barely makes a difference.

I drive a 91' astro van with sound deadener coming in a couple of days. The cabin is HUGE in this thing and flexs more than a circus freak with little effort. Could the amount of air in my cabin have a lot to do with my low Db output?