Bryon, Re Room treatments/tuning, like everything else in audio, ain't a Sunday walk in the park with a pretty girl.
Depends on what you are trying to change to create 'warmth'. In this post I will assume that 'warmth' means unemphasized highs with or without a corresponding wide but small boost in the lower mid-range thru to the upper bass. Room dimensionally induced issues? Set up issues? Equipment selection issues? Treatment selection absorption/dispersive panels, traps, etc, all of which require careful selection given the source of the 'problem' are critical.
For example, excessive sounding highs can be caused by equipment types or positioning relative to reflective surfaces. They can be controlled/reduced by speaker location, orientation, or using sound dispersion or absorption panels.
But, as in the case of absorption panels, a common type of treatment recommended and used by audiophiles, if the materiel used covers a broader frequency range than needed, i.e. you need to knock down a 5K peak but use materials which are absorptive down to 1500k you will have dulled down an otherwise well balanced mid range. This might enhance the sound of the lower mid range/bass to some folks but not to all.
Conversely if you have an upper mid-range peak, a very common problem in speaker and electronics, and you try to damp it with absorption materials you loose the highs as well as the mid-range peak. For me that removes specific absorbers from the list of possibilities unless you can figure out how to compensate for the unwanted change. And so it goes with a just few room treatments but set up problems as well as speakers and electronics share the same issues. How to get balance in your room? Even the experts often can't get it right.
So, to my point (finally). Tubes. If you have speakers appropriate to your room and to tubes in the first place, and these speakers have a reasonably good sense of 'natural' resolution, by using tube equipment and carefully using (rolling) tubes therein to get you to your sonic goals, you can tame common HF problems and even add some bass /lower mid range boost (that warmth you are looking for?).
The possibilities with tube equipment seem as endless as the frustrations experienced by many in the implementation of tubes, especially by those who aren't all that dedicated and like quick fixes, or miracles. One of the things that I would always recommend because of the learning curve involved is to keep it simple, even knowing that ultimately someone might want an all tube system. For example, there are quite a few good integrated tube amps now and are an excellent place to start as opposed to introducing separates and making a mixed system.
And, FWIW, realizing that there exists those who will vehemently disagree, with ss stuff you are excluded from changing its sonic signature significantly, keeping it off the horizons for adventurers. Wires and little black boxes can only do so much.
But I digress and apologize for going off point and getting on a box with such simple observation
