I read somewhere there is an upper limit to impedance to what the PC can drive (600 Ohms).
This kind of idea spreads due to the "copy and paste first answer on Google without actually reading the webpage or context" effect.
People are often talking about high-impedance headphones which need high output voltage. And 600ohm is one standard impedance for high-impedance headphones. And the computer soundcard is often unable to provide enough voltage. So the statement often gets corrupted into "soundcards can't drive 600ohm headphones" when the correct statement should be "soundcards can't drive headphones with low voltage sensitivity". Sensitivity, btw, is how loud your speaker gets at a specific voltage.
Typical active speakers have input impedance of 10k or higher.
So in my original setup the sub would be completely ignored, i.e. no current at all passing through, therefore no bass. It is as though only the speakers would be present. I could revert to my current setup whereby the sub and the speakers are independently connected.
Passive speakers have pretty good voltage sensitivity btw. It literally says in their specifications: 85dB (example) @ 2.83Vrms. With the 2Vrms that a computer soundcard can output, this is sufficient to sound like a dishwasher. To get the same amount of voltage sensitivity, your active speaker or subwoofer would need to be turned up to near max volume. Try it and see if the volume you get balances out.
I've used headphone amps to power speakers too.
The soundcard or headphone amp's ability to output voltage is not a problem. The ability to output current is.
Now, if you're thinking of Ohm's law, and how current and voltage relationship must obey Ohm's law, you're correct. So when an amplifier cannot supply enough voltage or current? It also stops supplying the current or voltage so that Ohm's law is preserved. Hence, it clips, or distorts. Since soundcards have low output current, the max voltage it can supply into speakers before distorting is also low, and follows Ohm's law. You can still get a comfortable 50-60dB out of it in most cases, although at elevated distortions (Not due to clipping but rather due to the lowered impedance, but that's a story for another day.)
BTW a stereo amp can be bought for less than $100.