Originally Posted by Jim C
As I recall bot Tom and Chef have reported that they hear a subtle difference in tone and not a night and day thing.
It would take a while but it is searchable.
+1 In my A/Bing of those heads, much more similar than different, and I find the MBFusion one of the least 'hybrid' sounding heads, even with the three tubes. A little less 'screaming' in the upper midrange versus the very bright/aggressive MB500, but nothing that you can't pretty well match with EQ. I did 'prefer' the MBfusion a smidge for that reason (the somewhat more controlled top end) and that big of 'edge' removed from the inherent tone), but again, with EQ, I got close enough that it didn't really matter.
My rule is to avoid tubes at all costs unless you can really hear the impact (especially an amp with three of them crammed in that small space). IMO again there.
To the OP, I would just keep that MB500 and be happy!