A multi-tapped tube amp will deliver maximum power to the speakers when the speaker's impedance is correctly matched to the appropriate output transformer tap. This may or may not be the tap that also sounds the best with that particular speaker; for instance, the 16-ohm high efficiency speakers I built for my parents sound best on the 4 ohm tap of their JoLida tube amp.
Now the panel of the InnerSound Isis is going to be a very unorthodox load. In the crossover region (1 kHz as I recall) the impedance will be fairly high - perhaps between 50 and 100 ohms (I don't know the particulars on the Isis). The impedance will decrease with increasing frequency, and bottom out around 1.5 ohms in the high treble. It's not real straightforward what number should be picked as representative of the panel's impedance. Therefore, it's also not real staightforward which tap would result in the best power delivery, nor which would sound best.
My suggestion is to try 'em all. I have several customers using multitapped tube amps to drive electrostats, and there is no clear trend among them as to which tap works best. Try one tap for one channel and another for the other channel, and A-B compare using a mono source (or using a couple of Radio Shack Y-jacks to get a mono signal to both channels).
Duke
