I had a highly modified Oppo used only as a digital transport (all audio/dac board removed e.t.c). One of the mods was an upgraded clock - but not just that - a unique dedicated linear power supply just for the clock alone. This hugely lifted the performance of the transport. I was running it spdif out to an external DAC.
Interestingly the clock was a SAW (Surface Acoustic Wave) clock, not an TCXO or OCXO. The people behind this mod were adamant that SAW clocks were superior in sound quality to OCXO clocks. I have never seen anyone else in the high end audio industry even say they have tried SAW clocks.
Unfortunately the Oppo mainboard itself died and all those mods (like a full on R-core LPS powering the mainboard) turned it into a paperweight. It was a way better sounding transport than any entry or medium level product you can get now.