It sure sounds like your tweeters are dead.
Do me a favor, run audyssey, and if the chirps sound like crap that is a dead give away your tweets are fried. Assuming you are still covered by polks 5 year warranty, print your invoice and ship it and your tweets to polks repair center in Vista CA (exact address is on the polk site), you dont even need to call them. Be sure to include your return address and name though. Should take around a week turn around time.
Regarding the power your receiver is rated for and what the 70's can take, you must learn that too little power is way worse and can damage speakers very easily then too much power. Your receiver is rated at 100 watts per channel 2 channels driven when running 8 ohm speakers. This is generally more then adequate power for most average listening volumes in average to small rooms, however sometimes certain peaks will require more then what your receiver is able to put out and it will clip the signal and damage your tweeters....sometimes you wont even hear this is occurring. Do this enough times and you end up with a dead tweeter. Running more then 2 channels drastically reduces the available power output of most multi-channel avr's. The closer to reference level you listen and the further away your listening position is from your speakers will also requires more power to be available.
FWIW, I dont necessarily think that the monitor series tweeter is overly weak, however because of the inexpensive nature of acquiring the 70's, there are a number of people pushing them with inadequately powered entry level receivers attempting to achieve reference level volume at too great a distance/too large of a room. Step up to a decent mid level or better avr from any of the usual manufacturers and you will likely never have a problem with clipped peaks damaging your tweeters, assuming of course your room isnt the size of a gymnasium
. I recommend mid level avr not because they are rated at higher watt per channel ratings (most are apprx 120-140 watts per channel), but because they often have better power supplies and can handle peaks better then entry level avrs with weaker power supplies.
On a lighter note....I once killed my tweets on my 70's by pushing my denon 1611 to +8
on the dB volume scale with my listening position at apprx 8 ft from the speakers....lets just say too many adult beverages were involved:p.....interestingly I dont recall hearing painful amounts of distortion or clipping, but the next day when I turned my system on I knew something was wrong.