So is a computer the source, that connects to the Sony receiver?
So the optical is connected to the computer's on-board audio?
what model is the Sony receiver?
The Asus Xonar DX or D1 sound card, used $50, comes with the CS4398 DAC chip.
Which should be a fair bit better then the DAC chip built into the motherboard.
Plus the Xonar DX/D1 comes with Dolby Headphone surround sound.
Just run an analog cable (mini-jack to RCA) from the Xonar to the Sony.
SMSL SD-1955 DAC, $100, ships from China, AD1955 DAC chip,
use an RCA to RCA cable, between DAC and sony
HRT Musicstreamer, USB DAC, $150
ODAC, USB DAC, $150