By "bridged" do you mean the Dynaco-design parallel connection of the inputs, driving both amplifiers simultaneously with identical signals and paralleling the outputs? If so, that is an inferior method that works poorly because...among other reasons...it mixes together the feedback signals that are supposed to correct each channel's "errors" separately; so that it corrects NEITHER channel.
It also does not result in a doubling of output power as one might think, because the power supply of the ST-70 is not sufficient to support that high a combined output. Compared to this, a Mark III or Mark II would be preferable. I actually prefer a conventional single ST-70 with its nominal 35 watts per channel, to two of them connected in this parallel-connected so-called 70-watt hookup. More is lost by this mode of operation than is gained by the added output power.
Does this mean I think Dynaco was wrong to provide this form of mono operation on the ST-70? YES!
A PROPERLY "bridged" operation of a stereo amplifier involves inverting the phase of one channel and taking the output across the two "hot" speaker outputs, with no connections to or between the "common" output terminals. It is most often found on car stereos where it permits a higher output power than would be available from a conventional amplifier on a 12 volt power supply. Some audiophile-grade amplifiers have used it successfully, too. Either way, to get the best results, you need a phase inverter stage for one of the stereo amp's inputs, and it must obviously be of superior quality.