Hello Wes!
As Shannon told the best uprade is the OPT and the James 6112HS is a great OPT for GSG
My own upgraded parts are the resistors (Takman 1/2W ,Kiwame 2W and Vishay NS ), the caps (Nichicon FG Fine Gold/KZ Muse,KX for the High Voltage) and Mundorf Suprem as coupling caps (but Orange Drops are good too!).
Not a very expensive upgrade that sounds nice
Great amp for small expense (Thanks Shannon,great Job
)
Sorry for my english but i'm French (nobody's perfect
)
Best regards.
Pascal.