wrote:

I have my eye on a pair of Rotel RB06 power amps. They are stereo, but
by oving a few jumpers can be used in bridged mono configuration.

However, although in stereo mode they can drive 4 and 8 ohm speakers,
apparently in bridged mode they cannot be used on anything below 8
ohms, ruling out their use on my Dynaudios.

Can anyone briefly explain why this is the case?

In bridged mode, with a 4 ohm speaker, each amp 'sees' a 2 ohm load.
One channel goes positive while the other goes negative and vice versa
so the voltage across the speaker is twice that in normal mode.
The current through the speaker is therefore doubled.
So if each channel produces its usual voltage but twice the current,
then the load impedance it sees is halved. Ohm's law....

--
Eiron.

May contain traces of irony.