audible distortion and tube gear?

As you can imagine there are many types of distortion and Ralph Karsten ie Atmasphere can give you pretty much the lowdown on this.

You need to distinguish "distortion" which the ear likes vs. the kind it doesn't. The ear likes octaves, fifths, etc. I googled it for you and came up with this.

Either of the below links will supply an info that will assist with this inquiry.

I'm sure the content will be challenged by those with their own interpretations of the data.   

https://www.sereneaudio.com/blog/what-does-jitter-sound-like

If you think of distortion as a type of noise, then yes, you could say that if you start with a high amount of noise to begin with you are going to have a harder time, but not impossible, hearing low amplitude noise that's in addition to the original.

On the other hand, our ears are pretty good at recognizing differences, so that's why I'd say it's harder, but not impossible.

A bigger issue is the room acoustics, IMHO.  Clean that up and you can hear a lot of things you could not hear before.

Either tubes or solid state can be made to sound softer, more polite, warmer, etc. that frequently comes with a masking or blurring of detail usually most notable, but not exclusively, in the treble region where you’ll hear muted dynamics, air, reverb trails, etc.  I could see equipment that is voiced this way masking certain types of distortion and may be why you might be hearing minimal differences.  Conversely, equipment that is designed to be hyper detailed can accentuate distortion, so as with most everything in audio the key is finding the balance that sounds best to you with the music you listen to.  Hard to say given your description and lack of other pertinent info what you should be hearing, but if you share what equipment/cables are in your system and what CD you’re using you’ll likely get much better and more useful insights here.  Good question though.