I think you may have a heater wiring problem, possibly a socket wiring issue.
If the tubes were getting low heater voltage caused by something sucking too much current out of the heater winding, it could account for the fuse blow and also for the dull sound. Tubes with too-low heater voltage do change tone a bit - not enough electrons going. But the tubes last a long time.
The fact that a 6.3A fuse blew, then an 8A fuse blew is a red flag. Something is eating too much current. A useful diagnostic would be to measure the heater voltage on each tube and to measure the current actually flowing in the heater winding of the power transformer. These are skilled-tech-only jobs because of the electrical hazards of working inside a live amplifier.