If you are referring to paralleled output tubes what you get is more power.  What you loose is transparency.  In my experience 3 sounds better than two and sounds better than 4 for reasons I haven't quite figured out because compared to a single tube they all suck.  Push-pull on the other hand is two tubes, but they are in series and only one at a time is heard so as long as the phase inverter has no notch distortion in the rated power band, single-ended transparency is possible.

Getting to your subject line, series bridged output tubes means each output tube is single-ended into a transformer and those transformers are connected in series with no connection to ground.  In this case the amp sounds nearly identical to what it did running in stereo, with no real loss of transparency.

Steve