As to the reference to adding a cable containing a passive filter. Clearly, adding a filter will affect the sound because it limits, attenuates, or shifts in phase the frequencies being transduced by the speaker. Referring to such as a "cable" is not accurate but so long as the manufacturer makes it clear that filtering is being added, I see no ethical issue.
This brings up what I believe to be two broad and different approaches, or goals, of home audio. (1) A goal of achieving the most accurate sound reproduction. I.e., the sound should be the same as the sound recorded on the medium, and ultimately, the same as the sound that existed at the time of the recording (this last nuance only comes into play when dediding on vinyl vs cd vs whatever other medium is or comes available). (2) A goal of achieving a sound that the individual finds most pleasing. Part and parcel to this approach is to treat the audio system as a musical instrument in and of itself. In my opinion, the popularity of tube equipment is driven by adherents to this approach.
I beleive that both approaches are valid. Just different.
Inserting a generic filter between the amplifier and the speaker drivers or crossovers is an example of the second approach. Choosing a generic filter- one not made for the specific speaker - is not consistent with the first approach. Inserting a filter between the amp and speaker (which in this case is equivalent to changing the properties of the speaker) could be a step in the direction of better accuracy, but in such case one would need to design the filter for that particular model speaker and to an extent, that particular model amplifier. At that point, it makes more sense to just design a completely custom set of speakers and place the filtering either in the speaker cabinet or in the amplifier.
