75 ohm V.S.300 ohm and S.Q.

No problem at all. The "characteristic impedance" of the cable, in this case 75 ohms, is not relevant at the frequencies of analog audio signals. It would be relevant if you were using the cable to carry digital audio signals, in which case a mismatch would not cause damage but would probably impact sound quality.

Not sure what your reference to 300 ohms refers to. If that is the output impedance of the dac, again the "mismatch" to the 75 ohm characteristic impedance of the cable is not relevant at audio frequencies.

Regards,
-- Al

most all RCA cables are 300 ohm

Not sure where you got that from, but it is certainly wrong. Most rca cables utilize coaxial construction, and I am not aware of any coaxial cable that has an impedance above 100 ohms, with most being considerably less than that.

Regards,
-- Al