Mmm good point. I tried my earbuds into the audio out on the motherboard, and they are fine L/R balance-wise.
Plugged my speakers back in to confirm, and sure enough its back to 10/90 L/R balance. As a test, I plugged in my earbuds to the audio out on the Left speaker (M-Audio AV32's), and the same 10/90 L/R balance issue is occurring, making me think its a circuit issue in the speaker's circuitry, rather than the Left speaker itself being compromised in the cone or similar bits.