If you put the speaker inside the cabinet, you get little benefit from the fancy features and audio quality. If you mount the speaker itself to the cabinet (remove speaker from plastic case, install in marquee area for example), you lose any benefit from the enclosure the manufacturer designed. If you mount the speaker to the side of the cabinet, it generally looks ugly. If I were going to spend money on speakers, I'd get a pair of bare speakers and a cheap amp to drive them. I'd probably still use a PC subwoofer since they are fairly cheap.
Most arcade games have crap sound that a PC speaker (you know, the 1.5" one inside the case on your XT system) could reproduce acceptably. Having a set of cheap amplified PC speakers (~$20 range) gets you a huge benefit in more recent games (they didn't have a subwoofer and PC speakers are probably better quality). The most modern stuff can't be run anyway.
If you run music through it, be realistic about what kind of quality you will get out of your system depending on how you mount things. I get really nice sound with my driving cab with a cheap pair of PC speakers with 4" Atari arcade speakers hacked in. I doubt paying another $500 would get me significantly better sound.
If you like the sound, good for you. That's all that matters anyway.