At the risk of redundancy, let me suggest taking a look at the paper Atma-Sphere linked to:
http://www.atma-sphere.com/papers/paradigm_paper2.html
Briefly, different amplifier types behave differently when they see a non-resistive loudspeaker load. Most solid state amps approximate a constant-voltage source, and these are the ones who double their wattage output into 4 ohms and halve their output into 16 ohms. Many tube amps approximate a constant-power source, putting out roughly the same wattage into 4, 8 and 16 ohms.
So, let's assume our nominally 8-ohm speaker has a 32-ohm impedance peak in the crossover region. With a drive level that equals 1 watt output into 8 ohms, a solid state amp will put out 1/4 watt into that impedance peak, while a tube amp will put out four times as much power (1 watt) into that impedance peak. So the tonal balance will change significantly depending on amplifier type, and which type of amp works best depends on what the speaker designer had in mind.
Speakers whose impedance curves are very smooth (Maggies come to mind) work well with either type of amp, provided it is powerful enough.
The argument in favor of zero global feedback, soft-clipping, class A tube amps is a more complicated one, and probably doesn't belong in this thread.
Long story short: Amplifier to loudspeaker matching matters. If this is a subject that's important to you, check out that paper.
Duke
