Philippe, both of these points have been discussed here on several occasions. The lower impedance switch or menu setting should never be used, despite the language in the manuals. It's a UL required fire-prevention measure to make overheating less likely. It does this by limiting the voltage output, which as a result limits current and power because of Ohm's Law(maximum power is cut roughly in half). So, the possibility of overheating is reduced, but at the expense of reducing the maximum designed performance.
Again, despite any manual language, the DTR-70(and others making similar claims)can't be bi-amped. There's only one amplifier with one main power supply section outputting through several channels of output transistors which act as valves to send each speaker the amount of voltage required at any instant in time. Connecting a speaker through two sets of output transistors rather than one doesn't double the available power, as some imagine, or increase it by any amount.
