Pretty sure Nick recommends 8 to 9 after displacement, suppose a guy would just have to screw around with one until they find what they like....I ran a 21" warden in 7.5 cuft tuned to 34hz on 3.5kw and it sounded pretty damn good.
I was told 8 after displacement i believe. Thats why i built my box to be like 9.65, i had it exactly at 7.95 after all displacements.Pretty sure Nick recommends 8 to 9 after displacement, suppose a guy would just have to screw around with one until they find what they like....
thisWhy do you think you need to go to a 21? Why not rebuild it to a 15 or maybe 18?...it seems unlikely you have room to do a 21" sub