Quote Originally Posted by MusicHead View Post
Be aware though, that the Sierra tower have 4 Ohm nominal impedance. The lower the impedance, the more current the receiver has to provide. ...

Dave or somebody else from Ascend can comment on the actual impedance profile of the Sierra Towers, as 4 Ohm is the nominal value. Same for the recommended distance from the wall, although a lot depend on the room acoustic. Generally speaking, all speakers benefit from some distance from walls, rear ported speakers even more so. Putting them too close to a wall would most likely make the bass too "boomy".
This is incorrect, Unfortunately, there is still much confusion about what "nominal impedance" actually is. The Sierra Towers have a minimum impedance of 4.2 ohms. Minimum impedance is not nominal impedance. The term nominal is confusing - it should really be average impedance. The average impedance of the Sierra tower is 8 ohms. 4 ohm nominal speakers will typically have minimum impedance dips below 3 ohms. If you examine the impedance measurement, you will notice that for 5 octaves, impedance is fairly steady at 5-6 ohms, and for another 5 octaves, impedance is steady at 9-10 ohms.

The IEC definition of nominal impedance would categorize the tower as a 6 ohm nominal impedance speaker.

However, it is actually more complex than that as one must also examine phase shifts in combination with the low impedance areas, which in the case of the towers, phase angles (areas of higher reactance) are in combination with higher impedances.

The towers are an easy load for basically any modern AVR, regardless of what the rated impedance of the AVR is. To this date, and with thousands of towers sold -- we have yet to hear of any overheating issues or turn-off problems with a receiver or amp, regardless of its wattage and/or impedance ratings.

hope this helps!