You are looking at this as if these things are modular. As if purchasing a power amp, a stereo tuner, a stereo cassette deck, equalizer and then connecting them all together with rca cables. Or, as in computers where you can purchase a new video card, hdd controller, adding firewire card to a computer without, etc.
In the previous examples, these "modules" are designed as such, with fairly standardized hookups. However, if you want to carve out a section of a boombox, then you need to understand that the circuitry would only have been designed to work with the circuitry designed in the rest of this particular boombox and may not necessarily work in another application without modification, addition of buffering circuits, etc. Otherwise, you could introduce group loops, mismatched impedances, noise, loading down of circuits, etc. This isn't to say that it may not integrate "easy" but it might also be a nightmare too. The only way to know for sure is to analyze the circuit or try it, After all, who's to say or know what the inpput impedance for that module would be, and the output impedance of the new signal source. Or whether this meter has a built in internal buffer amp, uses opamps or old fashioned transistor drivers?
Bottom line, not trying to discourage you but this is something that you'll have to figure out on your own unless someone here has already tried and hacked this using your exact same module.
