Yes, the horn can be EQ'd actively or passively. You'll want to...

For a bass box you might also look at the Onken box. It works very well with the 416. Should do OK with the 515. GM might know. I can send you the plans, if you don't find them. No horn flare, so easier to build then the Altec box. Looks cool, too.

The Onken looks like this. Onken Box