Michael,
If changing the tubes does not bring back the volume, and you are not familiar with testing electronic components, it may prove difficult to solve the problem yourself.
You are correct in stating it should be louder than what you describe that it is. Even cheap amps were more than loud enough until Elvis came along and R & R tried to deafen everyone by playing horridly loud.
If you are at all adept at electronics, I will share with you the following.
In my years of servicing amps, the most commmon cause of amps decreasing in volume, was resistors increasing in their rated value. So if you have an ohmeter, simply measure them (if you know how to read color code), and see if one or more has increased well beyond their rating. Make sure unit is unplugged. Ignore small changes from their rated value, since many resistors were rated + or - 20%.
There is a reason for these resistors going bad. When electronics first came into existance, resistors AND capacitors (the mainstay of electronic parts) were made of materials that invited failure. And that failure was caused by moisture.
Today, that is rarely the case, since caps are coated with Mylar or other materials impervious to moisture intrusion. And resistors are coated and often made of a plastic material that just does not let moisture in. Incidently Allen Bradley was the first company to make a resistor that did not go bad (assuming it was not overheated).
So it is rare for them to go bad nowadays. But on older amps, this was not the case. As such, resistors were prone to increase in value as moisture permeated them over time. The most severe moisture occured as the amp cooled down AFTER being on for a while where condensation was a real evil.
So again, if you do have an ohmeter and know color code, I would simply measure each one you find. If you find one that is a great deal higher than its rating, replace it.
While capacitors can cause low volume in certain instances, usually they don't. They usually cause hum if they go bad, unless they are "cathode bypass" caps, then they can cause a substantial fall off in volume. It is very hard to find a cap that is shorted or "leaky" (partial short), without being an electronic's tech. Because of "parallel" circuits, most have to have one lead disconnected to measure them accurately, and this can get very involved. NOT recommended for anyone who is not an electronic's tech.
A good way to see if a cap has opened up and causing the probelm; is to simply parallel it with a good cap while the unit is on. If the original cap is open, you will know it instantly when you parallel it with a good one.
CAUTION: While lethal voltages are used in AC powered amps, it is very rare that anyone would actually be killed if they touched such voltages.
BUT, a powerful shock CAN be had by working on amps with the power on. So please use extreme care if you do work on them with it plugged in. If you have ever gotten across a 400 volt B+ line, you will wish you were dead for a few moments
Been there done that more than once, I can garontee ya!!
Good luck, and may Jesus bless you in your quests,
carl
