It could be that the e-caps are shot. If not, try the amp and get to know it. Guitar amps often have less capacitance in the power supply because this promotes an earlier onset of distortion which can be desirable in a guitar amp. Boosting the caps can change the character or the amp. On the other hand, if it's too distorted, maybe upping the capacitance would be a good thing. Something that is best to experiment with but there are some serious power supply limitations. Look at the capacity of the 6X4. The deluxe uses a 5Y3 rectifier which can provide just over twice as much current. The 5Y3 differs from the 6X4 in that it uses a 5V heater, this requires a separate power transformer tap. The 6X4 uses a 6.3 heater, same tap as the other tubes.
I'm not a fan of some cap cans such as the FP ones used in a lot of amps. You can fit separate caps under the chassis rather than use the cap can. Another alternative is to hollow out the existing can and put a cap inside or leave the can mounted for looks but don't use it.