My 1997 Vox AC15 has started having problems. There is a scratchy/static-y noise, and volume dips. After awhile the volume just stays down, about half of what it should be, and wimpy sounding. I've replaced the 5Y3 rectifier tube and the two EL84 power tubes. I haven't replaced any of the 12AX7 preamp tubes yet. Anybody experienced these symptoms before?