Ha! I should have waited for your answers!
I did in fact insert 3 out of 4 Electro Harmonic wrongly (into a Chinese amp). Now 3 out of 4 test bad in my tube tester. When I plugged them back in, this time correctly, the channel with 2/2 testing bad plays low, the channel with 1 out of 2 testing bad, plays at higher volume.
I guess I'm in the market for some cheap 300b replacement then :/