prolly bout the smallest i would try w/ an SX15 would be 3cuft tuned to 35hz...i dont like to tune a daily box any higher than ~35-38hz but thats just me.
SX15's like some space thats why mine is in ~5cuft daily. Its supposed to be tuned to 31hz...but for some reason, i peak at 28hz w/ my daily port in and do 138.6dB.
i'm doing a rebuild this weekend...also i'm building 2 (maybe 3)other boxes. one for compVr12's, one for 2 orion 10s, and one for 2 L512's. Looks like i'm gonna have a box building business going around here...i have to pic up 4 sheets of MDF friday, lol.
i built a box for a friends 2 audiobahn AW1206T's this past weekend...5cuft tuned to 34hz. I ran a speaker wire from my amp to his subs in his truck w/ the 2 12's and it sounded pretty loud...but my SX15 utterly abliterated (sp?) them on every level. SQ w/ the a-bahns wasnt very good at all, but he was happy w/ it....i'm gonna get him a hifonics bx2005d to put on them.