Looking at pictures of the inside of the First Watt B1. Just Bypass, jump the volume controls with one wire on each pot.
I am another that is confused. I am understanding that you would like to plug your turntable (phono) into this the First Watt B1. Outputs to your passive volume control. Which feeds your power amplifier? Question. Where is the RIAA de-emphasis accomplished? Output of a Moving Magnet cartridge is about 4 millivolts. That is not enough to drive a power amp.
Ohhhh wait. You don't have one of those turntables that has a built in preamplifier (taking care of the gain increase and RIAA) and maybe even USB outputs, do you? The buffering you are looking for should be in that preamplifier.
Ummmmm.....