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. Check and see what value your Plate resistors are on your 12AX7's. You can try anything from 56K to 220K. The 220K's will extreamly monterous distortion. Most of the "sought after" and classic amps use 100K's for the plates. I tend to favor these.
2. You can also use different values for your cathode resistors. The most popular values are 820ohm, 1K, 1.5K, & 2.7K.
3. If this still isn't enough gain place a .22 or .68uf 25volt cap across the cathode's resistor.
3. Try a 1K & 820K tied together with a .22uf 25volt cap on pin#3 of the first preamp tube in the chain and then put a 2.7K resistor with a .68uf 25volt cap across it on pin#8 of the same tube.