The amp has a switching and easy bias setup on the back making it very simple to set up for tube swaps.
So here are the findings:
(#1) JJ KT77 (Slovakia): took a lot to get biased correctly, not plug and play at all. good bass, pretty well defined. very high output feel, had to turn levels down I'd be afraid of this tubes longevity.
(#2) Teslovak 6550 (China/Shuguang): Good bass response, not super bright and not expressive or super defined. still good tone medium output feel.
(#3) Gold Aero EL34 (JJ E34L): big bottom end, good all around tone, touch of glass. plug and play was very easy to go straight to playing.
(#4) Groove Tubes 6550 (Russian Sovtek): was plug and play and needed slight bias after the other 6550 where installed. Excellent big bass response. chug chug sound is evident. has good tones across the board. gets buzzy and thats with a humbucker.
(#5) Mesa Boogie EL34 STR 447 (Electro-Harmonix EL34): hollow, sounded meh, just was powering the amp and had no bottom end. not ideal for this head.
LOVE the tone of the tone of #1 but it is a worry about of the longevity, #3 and #4 are my favorite tone but #4 had this annoying buzz. Gonna keep #3 installed, overall best tone and functionality.