My SoundSticks II have been giving great sound since 2005, but I recently noticed that the left speaker is much quieter...

@gallina - You can often see swelling at the top of bad caps - see for example http://www.robotroom.com/Faulty-Capacitors-1.html . Your problem could also be a loose connection. I’ve had problems with the spade connectors inside the speakers corroding or coming loose (but it’s hard to access those unless the adhesive holding the speaker cases together has also deteriorated to the point where you can open the cases). Maybe you can try wiggling the speaker cables to see if that has any effect. It could also be a loose or broken connection on the audio input.

FYI, here’s an image highlighting the caps I replaced in red.

I am the OP, replying with the solution I found. The symptoms were very much as if a small resistance had been inserted in series with the left speaker. Peering at the circuit board, I noticed a pair of large-ish electrolytic capacitors near the connector that carries the speaker outputs, a common feature to remove the DC offset when driving speakers from a single-rail circuit. I also noticed that the cans were a little swollen. I guessed that the caps drying out or going bad could increase their effective series resistance, which could explain my symptom. I opened up the case and replaced both caps with new low-loss audio-quality 1000uF 16v electrolytics (same ratings as the parts I was replacing). It worked! The stereo balance is now perfect. I’d never heard of bad caps messing with the power of audio output circuits, only of them shorting out power supplies. But it seems like this is a real, if less dramatic, instance of the bad caps blight.

I found this useful pinout on Nick Brook’s site: https://nickbrook.me/blog/2014-08-11/repairing-soundsticks The R speakers are driven between pins 1 and 2 of the s-video 4 pin connector (the two furthest-apart ones, at the bottom, white and translucent in the cable). Pins 3 and 4 connect to the touch-sensitive pads on the R speaker for volume down (3) and volume up (4); you should be able to control the speaker volume by simply touching a bare wire connected to these pins with your finger (I think it detects mains hum or something). Note: Touching both volume controls at once puts the system into “mute” mode, which can make the system appear not to work. Simply touch volume up again to unmute. As you have found, the L speaker is simply driven between the two connectors of the RCA plug. The original satellite speakers consist of four 8 ohm drivers in parallel, so the amp is expecting a 2 ohm load.