Please explain amplifier output impedance

Impedence curves of amps is not usually part of the spec's. But, when you read reviews where they have been measured you will usually find them referenced. One of the reasons is that SS amps generally have very low output impedence, well less than 1 ohm. The effect of this is that the speaker will see a very flat electrical signal.

Tube amps are notorious for having high output impedence 'curves' (one of mine has a rise to 3.5 ohms in the mid bass frequencies. The net effect of this is in the frequency where this rise occurs I will hear a corresponding rise in frequency response in the speaker. That's one reason some tube amps sound warmer/looser etc than others, and most warmer than SS. I'm unaware of a loudspeaker spec other than its impedence curve which would be meaningful. What I think (and I really mean think) would be helpful is if your speakers impedence curve dipped in the same frequency that the amps impedence rose they might produce a flatter tone than either would on its own.

Clear as mud? With any luck Sean will read this and sign in with a crystal clear explaination. As I said, FWIW.

I'm not an expert on the subject but always heard that solid state amplifiers are low voltage, high current devices and work better with lower impedence speakers (8 ohms or lower), whereas tube amplifiers are high voltage, low current devices and work better with higher impedence speakers (8 ohms or higher; up to 16 ohms). Eventhough amplifiers put out more power into a 4 ohm load vs. an 8 ohm load, when the impedence of a speaker goes lower (4 ohms or less) it becomes an increasing difficult load for many amplifiers. Like Newbee pointed out, the impedence curve varies throughout the freqency range; some 4 ohm speakers, depending on their crossover design, may dip as far down as 2 ohms or even 1 ohm, which is a very difficult load for most amplifiers (except Krells which don't seem to mind). Hope this makes sense.