The advanced method if you cannot determine if it is tube related or something else (most likely it is tube related) is to pull power tubes. You can do one at a time if that helps. I generally mark my tubes with an I and an O for 100W amps. I am sure you could use a similar scheme with a 120W amp but with I, M, O. for a quad array, I mark the ones on the outside with an "o" or a dot on the Mesa label or on the plastic base (depends if I am using a black or silver sharpie pen). Inner pair get a line or an "I" just so you can keep the tubes straight if and when you have to pull them or if you want to swap the inner and outer pair if you use the 50W mode more often.
With one power tube removed, no blown fuse, you found the bad tube, the one you removed. Do not leave the amp on for too long or play it with one tube missing as this may cause the other tube of the pair to heat. It is okay to do this for diagnostic purpose. Just a short term to confirm the fuse did not blow. Removing all of the tubes is just a quick way to determine if the cause is something else.
Why it works in spongy mode and not bold, at spongy mode or reduced power mode, the issue may not be apparent until the voltage is increased at full power. There is not much of a voltage drop in spongy mode but enough to retard the current though the tubes as the plate voltage is reduced. You may have a bad tube (hopefully that is all it is.)