It's never a good idea to have a piece of equipment that has a high current draw such as a power amp on the same circuit as another that does not draw much current. The Ref 30 obviously has a low threshold where the power supply goes out of regulation. This is a design issue, and can be dealth with by having more filter capacitance to ride out momentary low voltage instances, or by having a higher voltage available on the secondary of the power transformer, or both. This increases the heat dissipation of the power supply however. These are all design tradeoffs, but ideally the power supply has sufficient capacitance and enough of a voltage buffer on the secondary that the AC dropout voltage is between 90 and 100 VAC. It's possible that your AC voltage is on the low side; it should be 120VAC. If your house has old, smaller gauge wiring, this will also drop excessive voltage as the demand for current increases, such as when a power amp is turned on, and it's filter capacitors charge.