You should teach yourself how to bias the 6550's. Then, if you check your bias on the present tubes, and it is considerably below spec, your tubes are getting weak. Bass response will become flabbier and weaker as they age, and they will lose some of the 'life' in the sound.
CAVEAT: You can electrocute yourself, or blow components on the board, board traces, and even parts of the board itself if your meter probes, or your hand, makes contact with the circuit board traces, or the leads of a resistor, etc.
There are two large resistors at the top of each large circuit board on each side (channel) of the amp. You really need to buy 'loop' ends to slip on the end of the pointed probes that most meters come with. You can buy cheap ones at Radio Shack. If you have a steady hand, and are careful, you clip the red loop to the lead on one side of the resistor, and the black loop to the lead on the other side of the resistor. You set your meter to mA and look at the reading. To change the bias, there are small blue Bourne pots have screwdriver slots, which you need a tiny flat blade screwdriver to adjust. Or, there is a tool that comes with each VT100 for this purpose.
It sounds complicated, but it is really a snap once you get the hang of it. You can practice on connecting the loops and getting the screwdriver in position, with the amp off and unplugged. You'll then get confident enough to turn it on, let it warm up 20-30 min. and adjust away.
The link suggested in an earlier post is good to look at, but it shows how to bias a VT-100 Mk II, which has only one pot per channel, instead of two. Also, these instructions concentrate on the extremely complicated biasing of the driver tubes, which you do not have to do on the Mk III version -- thank Audio Research that they ended that nightmare of a job! Hope this helps.