Bluetooth receiver board into 60's console stereo Tape In
Justme's mention of an op-amp was the keyword I needed.  Google informed me that &quot;op-amps&quot; are often used for headphone amplifiers.  I found a board-based one with a volume pot built in HERE, and put it between the bluetooth board's line out and the console's tape in.  Adjusted the volume to match other inputs wonderfully.