Here are some basic thoughts about changing voltages.
FIrst, as TONMANN posted several years ago, increasing the voltage does not increase headroom. I have noticed a change in the tone of circuits when doing this.
Second. The components must be capable of operating at the higher voltage. Electrolytic capacitors should have a rating that is at least 1 1/2 times the operating voltage to be safe. Most opamps and transistors used in these circuits should be able to handle 18V. However.
Third. The higher voltage will change the bias for all circuits that use transistors. Some of thee circuits do not have a way to adjust the bias voltage and may not operate properly.
Last. I do not think that the bass preamp operating at 18V will have any effect on circuits downstream operating at 9V. I suggest you try it first and evaluate the results.