Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. Replace the stock 6V6 power tubes with 6L6's and rebias. (Failure to rebias will adversely affect the tone and operation of your amp). This change will add more bass, increase the clean headroom, and delay the point at which the amp starts to break up. The 6L6 is a direct plug in replacement for the 6V6.
2. Replace the 5U4GB rectifier tube with a Solid State plug in rectifier. This will increase the operating voltage of the amp and make it run hotter. (Don't do this until you've replaced the filter caps "See the next step").
3. If the filter caps haven't been replaced in a long time, take this opportunity to take care of it. Any old or dying caps will fail from the increased load on the amp caused by the SS rectifier. (Use high quality replacement).
4. Remove the old speaker, pack it carefully, and save it. Replace it with a more efficient modern day speaker. (Jensen Reissues are a good choice). Just make sure the impedance/ohm value matches the speaker you removed.