Most of the switches are rated for several amps, but you won’t be switching during a high signal state, right?

In any case, if you’re not going to use a volume equalizing switch, you’re going to have to determine at which volume setting of the preamp does each brand of speaker produce the same sound level, say a -16db setting for ‘brand A’ and a -18db setting for ‘brand B’ to read the same acoustic level on a meter. As you go between listening to one brand and the other, you’ll need to stop the signal, change the switch(es), and set the volume to the appropriate level before resuming the comparison signal.

Do what you can to make the comparison fair, but don’t sweat too much. Even in a laboratory setting, it’s hard to set up ’scientifically’ valid A/B comparisons. At the end of the day, isn't what you're really after an answer to the question, 'which brand of speakers are my preference overall?'