Interesting diagram, and that means I’m out of my experience level. I’ve never seen an amp "bridgeable" like this before, but the high output impedance of the transformer probably makes it less frightening than a SS amp with feedback coupled output stage@erik_squires , @georgehifi When using tube amplifiers you can just parallel the outputs (and inputs) regardless of whether feedback is used or not. The Dynaco ST-70 instruction/assembly manual details how to do this.
Solid state amp are usually bridged (so the input to one channel is inverted from the other) as the output section of one amp is otherwise loaded by the other- leading to bad smoke.
For the former, ’monostrap’ is the correct term, for the latter, ’bridged’. The terms get conflated.
@psqt I take it then you have enough channels (four) to do stereo. Al makes a good suggestion as the inputs of the amps are also paralleled; if your preamp isn’t up to it you might hear some distortion, but I would expect that to be accompanied by reduced bass performance. I think something else is afoot- check all your tubes and make sure they are OK. If not the amp will lose delicacy and detail and may even distort in unpredictable ways. IOW I think you have a bad tube somewhere.