Stereo solid-state power amps can often be bridged (even if they already are push-pull), giving twice the power in one load of twice the impedance. Just inject the same signal inverted in one channel, wire the HP between both hot outputs, and tie ground outputs together.
I don't know if this is feasible with tube amps, would two independant output transformers, series-series connected work like one center-tapped ? On a schematic it seems OK, but in real life the magnetic flux will be separate
If it's safe, and without special PI trick, you'll obviously end up with a class-A push-pull amp with the sum of the power of the original amps. Exactly like your two separate amps, but in mono ...
If you're looking for more power, you'll have to change the bias for class-AB or B. But if you just need more presence, use some subtle stereo effect and place the amps far from each other.
I only give negative feedback.
