Thanks, Ralph. Here is a link to the ST70 manual, in which the relevant paragraph is on page 12:http://www.curcioaudio.com/st7_mnl.pdf
However, I think that what is being said is that to realize the 2x power increase, a 4 ohm speaker would have to be connected to paralleled 8 ohm taps, and an 8 ohm speaker would have to be connected to paralleled 16 ohm taps (assuming they are present).
If an 8 ohm speaker were connected to paralleled 8 ohm taps, the manual appears to say that a considerably smaller increase in power would occur, although distortion would be improved. That makes sense to me, and can be seen by considering the example of power delivery into 8 ohms, assuming constant source voltage but assuming in one case an amp output impedance of say 4 ohms, and in the other case an amp output impedance of 2 ohms (corresponding to paralleling the channels but without changing taps). That results in a 44% power increase. Amps with lower output impedances would realize smaller power increases.
I do see, though, that paralleling channels AND moving to a tap rated for twice the load impedance would result in twice the power capability, since the higher impedance tap presumably puts out a voltage that is higher by a factor of the square root of 2, and the paralleling of channels provides the current capability to support that higher output voltage.
So I believe that the bottom line is that for a nominally 4 ohm speaker the power can be doubled by using paralleled 8 ohm taps, but it can only be doubled for a nominally 8 ohm speaker if 16 ohm taps are available.
At least, I THINK so :)