Depends on how you are preamp tapped. You want the input into the phase inverter to be shunted to ground at the same time the tap has a jack in it. You also want to remove the speaker plug so that the jack shunts the output to ground. With both the input and output shunted to ground the power amp section will just sit idle. You can at this point keep them in for one type of voltage or remove them and raise the voltages across the entire amp. Of course you want to make sure your filter caps can handle whatever voltage increase happens. Check your voltages, know your amp. Lethal voltages involved so a competent tech is HIGHLY RECOMMENDED.