Keeping things in perspective.. that "cheap" sound card is likely to be at least as good as the original recording equipment was for LPs of the 60s or 70s..

I respectfully disagree. Sure a PC sound card is more technologically advanced than the mixing equipment they used at the time, but its analog stages are not necessarily higher quality.

Recording engineers from 30 or 40 years ago were not cavemen. They knew what THD means, they knew what a noise floor was, they knew how to make good clean recordings with what they had. And so did the people making the equipment. They knew how to build the equipment to provide the best recordings with the least amount of noise possible. Most PC sound cards don't have those kinds of specs. Ask anyone in the business of home recording. You shouldn't do home recording with a soundblaster (although I've done it in a pinch, I'm embarrassed to admit).