damping factor

But, particularly in the case of very efficient drivers, there is the issue of overdamping. Too much control over the driver can lead to a bass response that is less natural and not as deep as it otherwise could be. My Aleph 30 amp was great except that it was overdamped, causing an unsatisfactory bass response. I have since gotten an Atma-sphere amp with a lower damping factor that works much better with my speakers. As always, synergy with speakers is the key.

Good answers above. There is an additional factor that often needs to be considered, though. Damping factor and amplifier output impedance are inversely related. Damping factor, as usually defined based on an assumed 8 ohm speaker load, equals output impedance divided into 8 ohms. To the extent that the speaker's impedance varies with frequency, that output impedance will affect tonal balance, by interacting with the speaker's impedance vs. frequency variation.

Many electrostatic speakers, for instance, have high impedances at low frequencies, which descend to low values at high frequencies. A solid state amp, having a high damping factor and negligibly small output impedance, will produce LESS bass and MORE treble into that kind of speaker, relative to a tube amp. The tube amp, having a relatively low damping factor/high output impedance, will produce MORE bass and LESS treble into that kind of speaker.

Conversely, many dynamic (cone-type) speakers have low impedances in the bass and mid-bass regions, and higher impedances in the upper mid-range and treble. Driving that kind of load, a solid state amp will provide a stronger bass response than a tube amp, and the tube amp may sound excessively bright.

As was stated, synergy with the particular speaker is key.

Regards,
-- Al

I usually don't like talking specs, just because of issues like this... Think about how much damping factor measurements can change depending on where you measure from. What if you measure at the output, what if you measure at the speaker? How can two amps have great bass control with one having a damping factor of 50 or 100 and the other at 400 or even 1000.... For me, even though this is a very legitamate questions, its useless, you've got to plug the thing in and take a listen. By the way, Slew rate is just as important(the amps ability to recover after a hard bass note). Good Listening, Tim

additionally, here is a good write-up on amplifier output impedance & damping factor:
http://sound.westhost.com/impedanc.htm