They may be right, I am not sure how stable the Isis is they may dip down low, if they dip lower then 3 ohms, I'd feel more comfortable on the 2 ohm tap as well.
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It's really not a issue of 'safe' so much as sonic's, especially if absolute power is not an issue. Use the tap which sounds best to you. Try them all for that matter. I have a 160wt SF amp and I drive 4 ohm speakers with a minimum of 3.5 ohms off the 8 ohm tap. The highs are much smoother, and because I have 92db efficient speakers I have plenty of bass power available for any listing volume in my room. The bass is both full and tight. When I used the 4 ohm tap the highs suffered some. JME - YMMV.
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).