Discussion about the Korg PolySix synthesizer

No no...  basically, they are saying that the filter probably isn't bad.  There are 6 discrete filters, one for each voice.  If when you play 6 notes in a row (middle C pressed 6 times for example) they all sound the same (bad) there are two options.  One option is that ALL 6 filter chips are bad, or it's not the filters.  Since it would be pretty strange that all 6 filter chips would be bad, you probably don't need to worry about them.

Either the bad sound is coming from some sort of weird setting that is telling the voices to make the sound, or there is something wrong after the 6 voices are combined together.

Now if you only had one note of the 6 pressed that sounded bad, then it must be something on that voice, whether the VCO or VCF or related circuitry...