Slew Rate?

Slew rate and risetime are related terms that refer to how quickly the amp can change states to follow the input voltage to output voltage. Think of "acceleration." This can be a good indicator of how "quick" an amplifier sounds: getting the leading edges of transients accurately reproduced, for example. It also impacts resolution and detail.

A corrolary to slew rate is "settling time" and this is just as important to how resolving the amplifier can be. As the name suggests, settling time reflects how quickly an amplifier can return to its nominal state after passing a large voltage swing. Bumps in settling time will be reflected in the trailing edges of transients and can show up as ringing and added sibilance.

Spectral and Atma-Sphere are two other manufactures that publish a slew rate for their amplifiers. Atma-Sphere quotes this as "risetime" (using the same 600 volts/microsecond measure as Spectral), whereas Spectral makes a distinction between risetime and slew rate.
http://www.atma-sphere.com/products/ma2.html
http://www.spectralaudio.com/dma360_spec.htm

However, like any other measurement it's not worth reading too much into it because it's only one aspect of an amplifier's performance envelope and there are so many aspects of sound quality that we certainly differentiate in listening but don't know how to measure.
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Repeating something I have mentioned before, "settling time" tends to be the same as rise time, but really is not of interest in audio amps because musical sounds have leading edge transients, but die away gradually. This characteristic is used in the restoration of antique recordings. They are played backwards, and any transients that appear are identified as defects, and removed.

Another spec closely related to slew rate, and usely more widely published, is bandwidth. Amplifiers with wider bandwidths are faster and equate to larger valued (faster) slew rate specs. An amplifier that can reach 100kHz is faster, and would have a better slew rate spec, than one that goes only to 50kHz - the higher the number the better.

Current is the electrical property that is needed to drive speakers well, not voltage. Producing lots of current for a given power rating is part of what makes better amplifiers heavier and costlier. Current specs are rarely published, so one way to get an idea of an amp's current capacity (besides opening the amp and looking at the fuses) is to compare power ratings at different speaker impedances. An amp rated at 100W into 8ohms and 165W into 4ohms has more current capacity than an amp with 100W into 8ohms and 125W into 4 ohms. Please keep in mind though, that this only works if manufacturers aren't misleading when publishing their amp's specs. Product reviews where specs are measured are a good source of data for comparisons.

I have Magnepan planer speakers and use amps with high current capacity to drive them, with excellent results. Keep in mind that when choosing a high current amp, planer speakers need a certain amount of minimum overall power as well. I would recommend 100W as a minimum, as long as current capacity is there, but more would be better.

Hope this helps. :o)