First I listen to all types of music, but classical and country have some of the nastiest bass lines you will ever hear, especially if you like your bass in the 20's.

second, a ported box shouldnt be designed to be peaky unless its an SPL box. I know when I design them I design them to be flat as possible.

third, notice I mentioned MTX 9515's first. According to the MTX site it says 2.5cu per sub ported. I know thats a bit small, as the box I made for a 9515 was almost 3.5cu. I only mentioned the DD because I know it requires a much larger box than the MTX and those are the only 2 9500 series subs I can think of.

fourth, tuned low in too large of a box there is more chance of the sub being damaged. Both low tuning and larger boxes lend to more cone excursion, hence a better chance of exceding the x-mech of the sub.

and lastly, if the customer is happy thats all that really matters in the long run. I was just wondering, cause ~16.5cu (box after displacements) seems very close to IB. I wish I was home though and could graph it out myself. You wouldnt happen to have the response graph from the box design would you?