Are the old tubes still available? Put them back in, and see what happens, but I doubt that is the issue. What about this cap that "isn't made anymore"? All the signal caps are available, and the modern power supply filter caps differ in value by a slight amount that is routinely substituted and makes no difference in performance. 20uF vs. 22uF or 70uF vs. 89uF. A 60's Super Reverb is one of the best sounding amps ever made, if yours sounds harsh, something is certainly amiss. <looking at your bud>.