The relay attenuator description here says “To use it with any analog source: simply plug the analog output you want to control in the inputs of Allo Relay Attenuator and connect its output to your amplifier. This is extremely useful is your USB DAC does not have Hardware Volume control, and you use it with your Raspberry PI. Please note: to use the Allo Relay Attenuator directly on the Raspberry PI you need GPIO spacers, since the Allo Relay Volume Attenuator is not HAT compliant.”
However, the pin assignments on the Relay Attenuator do not match the RPi GPIO connector; it won’t work if you simply mount it on the RPi. It looks like you could use jumper wires to connect from the correct GPIO pins to J8 on the Relay Attenuator - power and ground are easy, I2C Data and Clock look easy. What about GPIOB15? Is that just GPIO 15, or does the Piano DAC introduce some additional logic?