IDA-6 was initially conceived for installation use, so it has 4 pairs of speaker outputs.
It was also designed with 2 pairs of outputs at higher power if the other 2 pairs are not connected.
IDA-6 doesn't have a bridging circuit. Here's the description from STA-9:
The standard method for bridging a stereo amp into mono employs a series of op amps (OPA) in order to invert the input signals to one of the stereo amp, thereby creating an opposite phase, which can then be combined to increase power. Conversely, the STA-9’s innovative bridging circuit utilises a design we call Single-Ended Direct-Inject Bridge Technology, in which the mono mode, in addition to acquiring more than double the power, sounds livelier and more dynamic.