Min, thought I would give you an update of your GSG 300B since it has come up a couple of times in the UX4 socket thread here http://www.diytube.com/phpBB2/viewtopic.php?t=2658.
Upgraded the solid state rectifier to include a RRFSR per ed http://diytube.com/phpBB2/viewtopic.php?t=2280, changed the tubes to Sophia Electric Princess 300B mesh plates, and swapped the coupling caps to .22uf K40Y-9's. All of these were done about a month or so apart with each of them making a worthwhile improvement.
Swapping the tubes from the Electro Harmonix made this amp allot more laid back and the the K40Y-9's brought it back up to speed.
I think I will play with bypassing the cathode bypass caps with solen fast caps next.