jj's are dark for preamp tubes and will get rid of the fizz. So will tungsols. I'd say buy like 2 of each and play around. But honestly, the best way to tame the harshness of a gh50l is to turn the treble all the way off, and use the presence control as your treble control.
However i wouldnt recommend using all jj's because that can have muddy results.
mind you, preamp tubes make barely any difference, so all of what i just said should be taken with that consideration.