Nonetheless, if resonance is as big an issue as some here claim, I would expect speakers with resonant cabinets to sound noticeably bad, and they don't. My thought was that maybe resonance only becomes audible above some critical volume level that I have never reached.
I strongly suggest you read:
The Modification of Timbre by Resonances: Perception and Measurements. J. Audio Eng. Soc. 36, 122-142, 1988 by Toole, Floyd E. and Olive, Sean E.
In this article the thresholds for human perception of resonance is researched. In the end it is found humans are extremely sensitive to this phenomenon that is very common in most loudspeakers. Not only that, but it is found that loudspeaker/room interaction actually decreases this threshold making resonance even more noticeable.
The reason you have likely never noticed this resonance is simply because you do not know what to look for when listening as you do not have an inert control to compare with.
Though I cannot say at what level a speaker must play to have audible resonance, think of how a resonance is audible; it doesn't sound like a rigging wine glass, it's a coloration that creates a change in timbre by the cabinet acting as a uncontrolled speaker diaphragm.
Resonance coloration is most often found in the midrange bands as that is where it is hardest to remove, but it is able to color any frequencies if given the chance.
Also, it is important to note that some speaker designers actually design there speakers to have resonance bands in the attempt to achieve a specific sound. While this might make sense this can cause poor interactions with rooms and source materials for example: if the source is recorded and there happens to be the same resonance bands the resonance will be over emphasized when playing that source.
So as you can see Joe resonance isn't necessarily thought of to sound bad right away, but when compared to a non-resonant speaker your thoughts may change. Another article by Toole delves into this topic:
Loudspeaker Measurements and their Relationship to Listener Preferences: Part 2," J. Aud. Eng. Soc. Vol.34, p.327