i have a mesa mark 4...i had it seriviced not long ago,and the tech replaced the reverb tube with a 12at7. the other preamp tubes are prob 5 years old. i havent noticed any degrading of tone..but im anxious to see if replacing them would improve anything. should i change the remaning 4 tubes? can i just buy any good ax7 to replace the other 4? the only complaint i have is a slight bit of hiss i get from r2 and lead.