Blackcat has it right. Im 99.9% sure that GG uses standard tuning for a majority of his songs. There are a few songs with drop tunings, but it's nothing like how Hendrix and SRV would down tune in ord ...
Sorry to say a tube change wont change tone that much. Even going from something like 6L6 to EL34 the tonal change isn't much. The charactaristics may change a little. For example 6l6's generally have ...
90% of tone is in the fingers. The other 10% is in the equipment. I get extremely similar sounds from my Vox-VT-40+ that I do from my custom Dumble clone rig. Sure the tube amp is way more responsive ...