Smallest size box for a 15"?

[sonikaccord]

Meaning, if I give 2 of the same subs 150W RMS, together they will make 3db more sound than one of them getting 300W RMS?

Correct. and if you give both subs 300W its a 6db increase.

[gloryaccordy]

I got a Rockford Fosgate TI 15 in what I think is a 2.0cf box... gonna power it with a... *sigh*... Crunch PZI1500.1 (got it for dirt cheap)

How is this sub for SQ? I have it on my Cadence now and it seems OK but doesn't really bump as high into the freq range as my old 12 or other subs I've had. Almost seems like there's a gap in my low end