Smelly softening woofer and midranges

I have noticed a warm, plasticy smell coming from a speaker in my system. The plastic cones of the speaker seem to be getting soft, sticky, and attracking dust. I'm running within the power rating of the speaker. Polarity is fine. I never run over half volume, certainly not reference levels. Sound seems ok. I'm wondering what might be going on and how to troubleshoot things. Any ideas?
Roger, how old is the speaker and what part of the speaker is getting sticky (e.g., tweeter, woofer)? If it's a woofer, try pressing it in with your hand (fingers spread touching the cone around the dust cover of the speaker) when the power is off. Any rubbing noise?
Ozfly, Speaker is less than a year old. Tweeter seems fine. There are two 6 1/2" woofers and a 5 1/4" midrange. Using your test method, the woofers were both quiet upon light compression. The midrange, however, made a rubbing sound-almost like the sound of a manual bicycle tire air pump, both on the in and out cycle of the test. The speaker's been off all day while I was at work. Nothing sticky noted now. Watcha think? Roger
Additionally, speaker sounds rather flat and veiled-certainly not clean sounding. Could be related, or perhaps an unrelated placement issue.
Would you consider unscrewing one of the smelly drivers? If yes, do so, DON'T disconnect it and place it on a stool (or any support). Drive the spkrs and check if the driver is heating up.
It sounds like you're overheating the voice coil -- but why is beyond me (dc entering the system? Coils shot??). As you say, you don't drive the spkrs too hard anyway...
It sounds like your midrange voice coil is shot or there is simply some dirt interfering with the voice coil. There should be no sound (I suspect that your midrange on the other side is quiet -- if it makes the same sound, then it might not be a rubbing voice coil but simply the sound of the speaker assembly being compressed). Since the unit is less than a year old, I would contact the manufacturer/dealer you purchased them from and ask what the appropriate next steps would be.

If I were a betting person, I'd bet you would need to replace the midrange speaker (or at least the voicecoil/spider/cone assembly -- if you do this, be sure to clean the magnet gap with folded masking tape before reinserting the coil -- send me an email for more details if you have to do that). When the voicecoil rubs as it moves, it will heat up and I believe your stickiness comes from the heat buildup impact to the speaker. You may have had the problem from the onset, but it could just be getting worse with wear as the heat damages things further and the damage then causes more heat. Or, as Gregm suggests, you could have had some dc going to the speaker.

In any case, I suggest you contact the folks you bought the speaker from to see if it is under warranty or if they have suggestions specific to your speaker.
Great ideas gang. I'll do more testing later this week. Since the speaker is still under warranty, I want to avoid any extensive tests that might void the warranty. Everything is pointing toward the midrange voice coil.
Do you or anyone else smoke in the same room with the speakers?
The speaker is in a non-smoking environment. I haven't had the speaker long. The previous owner may have been a smoker. I really don't know. What effect would smoke have?
Nicotene will coat the plastic cone making it sticky.