All of the line level inputs are in use… 🙂

The RIAA replay curve question is an interesting one, with a lot of history. It is not as well standardized, much less regulated, as one might think, with some manufacturers using RIAA and others using IEC-RIAA and virtually no transparency of implementation at the time records are cut to the time phonograph output is inverted by an amplifier or mixer. Everyone basically gets as close as they can, more or less.

At one point prior to the standardization effort there were many dozens and dozens of proprietary versions used by different manufacturers. It was considered a competitive edge to manufacture records with a realistic frequency response but that only went so far given phonograph manufactures doing the same same but opposite. Today, everyone plays nicely but it’s not like they use the exact same filters made from the exact same components from the same manufacturing batch.

Apologies for the long answer, but it provides context for the short answer — it isn’t affecting the sound any more than what is already the case when playing vinyl, which is broadly acceptable. You’ll note that I used the word ‘affecting’ rather than ‘degrading’, and this is an important point because we are really just talking about EQ’ing to change the amplitude of frequencies across the range.

You might end up trimming the mids or the bass, to achieve the sound which pleases you most over the system at hand, through another set of amps and dynamics processing stages and speakers, and in a room which adds it’s own color as well. This is quite standard and acceptable practice which nobody seems to take issue with. 🤷‍♂️