Just looked at the amp's owner's manual. It doesn't specifically use the term RMS. It uses continuous with an asterisk, leading to a statement about FTC ratings. I haven't gone into the FTC ratings system, but it's possible that it's the same as RMS (which, for all its faults, is the closest thing the audio world has for a gold standard).
The BIG issuse is that your amp is not designed to drive anything lower than 6 ohms (and only when a switch is actuated),according to what is says and doesn't say in the 120 page owner's manual. At 6 ohms, it can drive two channels to 120 watts. Based on what I know and have read, this would suggest an amplifier stage that is quite current limited... perhaps because of a small power supply or limited capacitor capacity.
Polk literature is rather coy when it comes to the RTiA9's impedance. All they say is "8 ohms nominal", which isn't saying a heck of a lot. Some speaker manufacturers manuals have diagrams. Many others list minimum impedance. Shame on apolk for using neither.
Regardless... A Sound and Vision *test* said that "Impedance reaches a minimum of 4.63 ohms at 5.3 kHz and a phase angle of –34.92 degrees at 105 Hz." Now 5300 Hz is pretty much in the middle of the audible range, so your amp will likely be spending more time there (this is an oversimplification) than at sub or very high frequencies.
So the problem is in matching the amp and speakers. It's not a power issue per se. It's an issue of a set of speakers that is asking for more than an amp of limited capacity can deliver.
This isn't particular to AVR style amps, but I see it more often in consumer-level AVRs sold thru big box stores. (Cue AVR fans who will now pile on to defend their honor.)
As an aside, I have a 30 WRMS amp that has no problem driving a set of 8 ohm nominal speakers with 88 dB/W/m whose impedance dips to 3.7 ohms at around 100 Hz. The amp is a high current design and rated for 4 ohm operation. It doesn't get hot even when playing at 85-90 dB.
So it's not power, but design and design limits.
