choosing a method
There are several methods commonly used for making box joints, including on the table saw, on a router table, or with a dovetail/box-joint jig. With the latter, the spacing of the "fingers" on the template requires a guide bushing and bit of a size appropriate for the jig. Note, however, that not all jigs are created equal.
In your application, another consideration is the size of the amplifier cabinet components. Large components can be a little unwieldy on a table saw or router table, since the cuts are being made with the component on end. Any wobble will likely result in a loose fit, or worse. Because of that, my personal opinion would be that a bench-mounted jig might be easier for you to use.
If you decide to go that route, I'd suggest investing in one of the better jigs that will also do through dovetails - either a Porter Cable 4200 series, or one of the Leigh jigs. If you do a lot of these, the PC OmniJig might even be appropriate. I think the higher price of the better jigs will pay you back in ease of use, versatility, and better-fitting joints in the long run.