My speakers suddenly sound 'muffled', especially the right one. What happened?

The problem

I have a desktop speaker system by Altec Lansing, model FX4021. The setup is:

PC -> control unit -> amp + subwoofer -> 2 satellite speakers

and the control unit can also feed earphones instead of the amp and speakers. This has been serving me faithfully since 2008 and over multiple countries and apartments...

Anyway, yesterday, suddenly, the audio started sounding weird. I don't remember what I was doing exactly then, but there was no trauma, or power outage etc. What does "weird" mean? Muffled somehow. Low-frequency overtones remains strong and pronounced, but "most" of the sound of music and speech is off somehow. As though it was a re-recording of someone speaking far from the mic. Or, I don't know, I'm not an audio expert.

My attempt at diagnosis

• I used this online pure-frequency player, and could not find a frequency range which was missing.
• I used this online speaker-test utility and found that the "muffling" seems to be mostly(/solely?) in my right speaker; but that speaker is not silent. I also used the frequency range test, and found the bass to be incredibly strong compared to the mid and treble, and that's when I've already used the control unit to weaken the bass relatively.

Some physical 'diagnostics'

• If I listen to audio using earphones, connected to my PC using either a PL jack or USB, the audio sounds fine (I mean, not the same dynamic range as speakers, but the same as it has always sounded).
• If I connect earphones to the FX4021 control unit, they sound just like they do when connecting them directly to the PL output on my PC. i.e. this sounds fine too.
• If I replace the PL / 3.5mm audio cable between my PC and the control unit, there is no change in behavior.
• Disconnecting the electricity to everything and restarting has no effect.

My question

From your experience, what is (or are) the most likely causes for this situation? And - is it possible to rectify with a reasonable investment of effort and money (e.g. replacement of components / a part of the system), or is the entire system just "a goner"?

More about my system

• The two sattelites are connected to the amp/subwoofer using Red+White RCA audio jacks.
• A copy of the manual is here.
• An image for you:

There is a very simple possible failure which is also simple to check and kind of matches your description. Loose connection anywhere typically leads to stronger bass because dynamic speaker impedance is typically highest in the bass part. By the Ohms law it means that bass is less affected by resistance in the circuit.

If you measure the electrical resistance of both speakers and it's significantly different it's almost guaranteed you have corrosion or broken solder joint or whatever else.