Is it just the Top Boost channel with the problem ? The reason I as is because V2 is usually the first tube to have issues in these amps and it is only used in the Top Boost channel.
Looking from the back of the amp v1 is on the left under the aluminium cover. you then have a further 2 pre-amp tubes and then the EL84 power tubes. (Tip remove the metal grill from the back of the rear panel of the amp and you won't have to keep removing the rear panel to change tubes)
V1 is shared by both channels, they use each half of the double triode respectively. This tube should last well as its not stressed much, it is the most sensitive in the circuit though so a good low noise tube is best. I prefer to use a TungSol reissue (russian) in here because they are bright and clean sounding.
V2 (the next tube along to the right) is only used in the TB channel. It is wired as a cathode follower which puts a lot of stress on the tube due to the large voltage between its cathode and heater (around 180v). It drives the tone stack. Many modern tubes don't last long in this position in my experience. I have had TungSol and Mullard reissue tubes fail within days in this position. I have found JJ and Chinese 12ax7B tubes to work well here.
V3 the last of the preamp tubes is the Phase Inverter that drives the power tubes. This should usually be replaces with every other set of power tubes as it does take a bit of a hammering but nothing like the cathode follower position of V2. I have used TungSol reissues in here without issues.
The power tubes are running as cathode biased and fairly hot, they won't last that long in these amps. It does of course depend how often and loud you play. It only takes one tube out of the four to go bad and cause the sound of the amp to change. If you still have the older tubes try swapping each one out. Don't worry too much about matching tubes, they don't stay matched for long in these amps ;-)
New tubes can fail very quickly if they have a fault and its not as uncommon as you may think. Just swap the tubes out one at a time and use a process of elimination to find the bad one. Remember V2 is only used by the TB channel.