First, I should say that beyond having some success with DeClip long ago I'm otherwise not at all experienced using Izotope Rx. I say long ago because I was gifted Rx5 Advanced when it came out. Of course, I'd want to know from actual long time Rx users, how much of a jump, percentage wise, in deClipping and deNoising power going from Rx5 Advanced to Rx10/11 would be.

But my biggest concern: I have numerous treasured pop and soundtrack recordings from the 60s on CD-most from the original labels who released the same recordings on vinyl way back then. I recently played them on a friend's high res speakers; see posts 15266, 15276.
https://www.diyaudio.com/community/thre ... 2/page-764

Most of them sounded compressed to various degrees. And some of those also sounded "congested". Sadly, the compression that was applied in the mastering room is forever baked into these recordings. But does applied compression in general add noise?

And did the compressor models in popular use in many of the studios in the US throughout the 1960s also add noise?

If yes, what's the likelihood that the usual amount of such noise would substantially degrade the overall sound quality-especially when heard through high res low distortion horn speakers?

But if Rx can remove that compressor added noise how much easier to do so for a newbie would it be using Rx10/11 than Rx5?

OTOH, might there be a special plug-in or some other restoration platform that would work better for this?

Compression doesn't add noise but it can raise the existing noise floor by decreasing dynamic range.

Broadband noise reduction software, including RX, tends to take some of the life out of the music in my experience. I usually only use it on intros, outros, or breakdowns where the noise really stands out. Even then it compromises the audio to my ear. Sometimes a simple low pass filter works better on certain passages. Sometimes I just leave the noise alone.