On the Fender amp, it could have a board built on an assembly line and with wave soldering with the lead-free type of solder, and possibly the plated-holes of a double-sided board. It needs more heat and doesn't readily flow the second time around. Desolder what you can first, then flow in some tin/lead, desolder, and maybe again another time or two, displacing the lead-free with the better wetting tin/lead rosin core.
Or, if you can get the tip of the iron on all three legs and pull the transistor out while the solder is melted, you can do the above to clear the holes for the re-install.
The lead-free solder can eat the iron tip, so wipe it good on a copper scouring sponge to get this stuff off the tip. Coat the tip with some tin/lead and wipe it on a damp sponge.
This PT topic deals with ICs on double-sided boards with plated-through holes:
https://www.paia.com/talk/viewtopic.php?f=29&t=48