I haven't had any upgrades to my Thunderfunks........yet. However, I would not hesitate getting Dave to upgrade either of my heads.
I plan to get the 750 upgraded next year, not from the need of extra volume or anything like that, I just want it beefed up a little.
I would suspect that the 550 could only be upgraded to the 600. I believe there is enough difference between the 550 and 750 platform that it wouldn't be possible to upgrade from a 550 to a 750/800. But that is just my guess.
I do know that the physical dimensions of the 550 and 750 are different.
MIA P & J, Jule Monique Rack, Crest ProLITE 2.0, Thunderfunk TFB750-A & 550B, Aggie 3xGS112, fDeck Series 3, Thunderfunk Club #35