Repurposing dual 18" cabs for single IXL 18.2.2

Hi Everybody.

I am hoping someone can help me with my proposed sub build.

I have a couple of empty sub bins that housed dual 18" woofers. They were originally used for commercial cinema duty and are each around 550 litres gross internal volume. Just to clarify, the 550 litres is measured without deducting the internal bracing volume or the volume of the 4 x 4" port tubes that are currently in situ. The bins are very well made and weigh enough to make a hand trolley fearful.

I want to re-purpose the cabs by cutting off the backs and reducing the volume to about 400 litres net. I will seal up one of the existing speaker holes and put an IXL 18.2.2 driver into the other. Does this make sense so far??

I have been reading up on a few threads in here that suggest 400 litres is a good size cabinet for this driver (15hz tune) so that should be quite achievable by blending these boxes momentarily with a circular saw. I don't want to throw these bins out and the work involved in volume reduction shouldn't be too difficult.

Now, I am a little short of info here because I haven't measured the internal volume of the 4 existing ports and wont be able to do that until tomorrow. However, I would appreciate some advice on what the port volume should be for this build and whether 4 x 4" ports of the correct volume (and properly flared) are OK to use. I think Steve Callas recommends 8" ID ports for these subs and while I can certainly refit an 8" ID tube, I am kind of hoping I might get away with using the 4" ones. Less work and all that....

I appreciate any assistance you guys can give here. Thanks.