A common failure point of most amps is the output transistors. There's at least one, usually two per channel. Granted sometimes they are integrated into a power-amp chip with more components. Yes you can have one channel blow out and the rest still work. On the other hand, could be a frayed wire, corroded connector, or as already mentioned the speaker itself may have failed.
Using a multimeter in AC voltage mode (20V range or so) you should be able to measure input audio signal to the amp for each channel, and output to each speaker, once you figure out the wiring pinout. With it playing audio you can deduce which wires are which based on using the fader (left/right/front/rear) controls to vary the signal levels which would vary the AC signal voltage a corresponding amount.
The speakers might not have much kick because with a sub they could have ran a high pass filter and let the sub handle more of the bass, reducing the output power requirement to the other speakers. Well that would just be lack of bass, if there is early distortion (at an unusually low volume) then the amp power supply circuit might just be worn out, dried up capacitors for example that aren't holding the power rails up during peak bass.
You might get lucky, might just be something simple and free to fix like a cracked solder joint or two.
