No led on one Polk audio RT300P (bottom piece), but the top piece is ok. Is the power supply or crossover broken?

check the diodes,for voltage stages.the unit may be getting power surges do to open(faulty) diodes

Many of these SUB amps ended up with Capacitor failures in the Power supply section . Find someone who works on amps or TVs to have a look see.

it sounds like its the cross over

Since they're not self powered I wonder how much audio power they were absorbing at the time or maybe how mouch distortion, as THAT kills speakers faster than clean power. The only things that can burn up are the crossover and the individual drivers. I can't find anything about a fuse, which almost certainly would be blown, or should have. A blown crossover probably wouldn't affect the woofers. You COULD access each driver and test each with an ohmmeter for dead short or infinity.

Could be the power supply but the transformer may
have opened up due to a "surge". Might have to contact Polk to see whats up and/or dealer as the
amp may need sent in for repair!

Define limp. No highs or no volume?

Logically, what one thing would affect both speakers? Logically, how would a crossover failure mute anything but the tweeter?

So, what happened just BEFORE this problem appeared?

I'd look at the electronics.

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Well the next step would be to make sure that all the settings are set to subwoofer yes, and set your crossover as well.  The amp usually has a crossover which tells the microprocessor which frequencys to send to the sub.  

The power supply or amplifier has a shorted component.

They're usually just a standard DPDT slide switch. Try Radioshack.