when updating the sms-1 (mine is still version 2.1) will a usb to rs232 adaptor work, or do I need a serial output on my pc?

do the sms rca outputs put out an extremely low volume? I'm having to turn my reciver WAY up to get the test tone to the 76-80db range - (at that volume, if I had a normal source plugged in - I would blow my ears or the speakers)...although this is at the SMS-1 default volume of 15.

Thanks for any input, as I'm just getting started with this.