Tubes for a Peavey VTM-120 head

Hey. I bought a Peavey VTM-120 guitar amp head recently. it looks like the
tubes are the same ones it came with in the 80s, cuz both the pre and power
tubes say peavey on them and both have silver tops.. How much of a
difference would changing the pre and power tubes make? I'm trying to get a
crunchy, heavy sound with mid-range bark. How much do new tubes really
effect the sound? For example, what pre (12ax7) and power tubes (6l6) should
i buy to get the sound I described above. The amp already sounds awesome
already, but i'm assuming i can achieve a better tone with new tubes. Any
suggestions on what pre and power tubes i should buy to enhance the sound
i'm already getting out of this guitar head?