@bache @knotscott @erik_squires
While it is true you can get more power out of a solid state amp driving 4 Ohms as opposed to 8 Ohms, the simple fact is that all amplifiers make greater distortion into lower impedances.
The distortion that generally shows up in solid state amps is of the unmasked higher ordered harmonic variety- to which the human ear is keenly sensitive! This means that the presentation is likely to be brighter and harsher, since the ear assigns that tonality to this kind of distortion. Distortion obscures detail; increasing it seems counter to the goals of high end audio.
In a nutshell, if sound quality is your goal your amplifier dollar investment is best supported by a speaker of higher impedance; if sound pressure is your goal you have a 3dB argument for going with 4 Ohms as opposed to 8.
4 Ohms has other disadvantages, for example the speaker cable becomes far more critical!
I've often heard the argument about how good a certain 4 Ohm speaker sounds; the counter to that is that same speaker technology would sound even better if it were 8 Ohms, not on account of the speaker but on account of the distortion being reduced.
We like to think that certain small increases in distortion is 'negligible' or 'inaudible'; such talk ignores how the human ear works, which is to say it uses the higher ordered harmonics to tell how loud sounds are, and so has to be keenly sensitive to their presence!