-DanF

Exactly what does the 400 Watts RMS rating mean. I.e. is it one channel driven, two channels driven, all channels driven?

First, a point of clarification. The term "RMS Watts" has no engineering meaning; however it is usually interpreted to mean Average Watts based on RMS voltage measurements into a reference impendence. As a practical matter, the measurements are usually done into a standard resistor. Klipsch's rating is based on this method with the following conditions:

- The system is hooked up to 120 V.
- All channels are driven simultaneously; the satellites into 3.6 ohm loads; and the sub into 1.8 ohms. In the case of the subwoofer rating, the 1.8 ohm reference impendence is derived from the minimum impendence specification (2 ohms -10%)for two four-ohm subwoofer drivers hooked in parallel.
- the satellite channels get 1 kHz input signal, the sub channel gets 100 Hz.
- the output voltage on each channel is recorded at the onset of clipping.
- The resulting calculation for each channel is added together to give the final result. If each channel is driven separately, you are likely to see voltages that allow up to 75 Watts at each satellite. Note that these are RMS values,and for sine waves, RMS is about 1/2 of peak power ratings.

If the amplifier were rated for peak values, it would be about 800 Watts. Peak Music Power Output does refer to milliseconds of signal duration and doesn't really present a useable figure for audio signal reproduction. Short term continuous refers to 30 to 60 seconds worth of full power, after which some heating will take place. Most program material, as you probably know averages 10 dB below peak values, so the amplifier is running relatively cool. Then, a relatively long duration crescendo would test the upper limits for this short but meaningful time. This is the pattern that music and other program materials follow, and so the amplifier was tailored for optimum performance under those conditions. A 30 minute rating would be somewhat less, but probably in the 250-300 Watt range.

The "onset of clipping" is around 1% THD. Not a lot more power is observed at higher distortion levels.
A few additional clarifications: There are some speaker protection circuits in the system, which would make independent measurements a little tricky; however sine-pulse testing would confirm these levels. Some audio gear is rated at long-term continuous as opposed to short term continuous. This component is rated for short term, which relates more directly to how audio gear is actually used (i.e., unless your program IS in fact 8 hours of sine waves, the difference between the two methods is not significant, audio-wise). The difference in hardware, between a system that would be long-term continuous (and this one) is about $50 worth of aluminum. Since this is a switching power supply, the size of the power supply has no relation to the power output, as it might in conventional audio gear. We're just going to have to get used to our audio equipment getting lighter weight, and higher performance...

The combination of efficient power supply and optimum power design is what allows a 400 Watt amplifier in such a small space in the first place.

The reference impendence of the amplifier is in fact different from the impendence typically measured on the speakers. The speaker impendence is typically dialed in by the system designer based on a number of factors including several independent of the amplifier (such as optimum level and headroom matching of subs and sats). In the case of the ProMedia systems, the satellite speakers themselves would probably get an independent impendence rating of around 6 ohms

>>What's the recommended placement in a 12 x 12 room to get performance that is as neutral possible<<

Corner placement almost always reveals the nominal character of the subwoofer. This is where the room effects are minimal, and the gain of the three reflecting surfaces is as uniform as possible. That said, if I were trying to knock down a 70 Hz bump in a square room, I would try to find a point along the diagonal that coincided with a null, to get the smoothest possible response. Could be two feet from the corner, could be five.