Yes, the "Line Out" has always been better in sound quality compared to the "Speaker Out" option. The latter is often noisy*, even with toned down volume, and sometimes, the "instruments" sound over-amplified with sounds that seem to come out of torn speaker cone.

But I don't think there were any standards of Line Out that the industry of PC sound cards adhered to then. Not sure though.

*One option to reduce the noise coming out from the Speaker Out sound is to let it pass through a dedicated audio receiver/amp with a low volume output, and let the receiver handle it. (I have a Sound Blaster 1.5 connected to my receiver (Marantz SR5200) - noise is almost eliminated and the audio quality is comparable to the Line Out.).

Took me a while to find the actual question in that post 😀

I don't know, but having recorded from many Sound Cards, I don't think there was a standard for line level output as they are all over the place between various Sound Cards.

Most cards also have a mixer chip, making it necessary to mute certain part (line-in, speaker, AUX, CD) for the best results.

The old 8 bit cards can potentially sound fantastic, the main problem is cross-talk / interference from the computer and being able to the hear the "computer thinking" sounds. This varies from machine to machine and in my SS7 machine I was able to do wonderful recordings (Some are on my YT channel) on a Sound Blaster 1.5.

So for Sound Quality it's more about picking Sound Cards rather than worrying about speaker out or line-out. My ESS Audio Drive for example is amplified, but a really quiet card. Same goes for the Yamaha based Audician. My Sound Blaster 16 card and AWE64 value are quite noise, regardless of type of output.

The Roland modules also have various levels of output level. The CM-500 outputs a very strong signal compared to other units for example.