Dynaco initially had preamps called the PAM-1 (I'm guessing Pre Amp Mono version 1) which used the sockets on the front to pass the signal (instead of the RCAs) and for power. After the invention of the PAS2 (Pre Amp Stereo version 2?) you would use the RCAs and that was pretty much that.
You also use the octal sockets on the front to measure the bias voltage and keep it in check.