Well, as you have found out, you have gotten many different pieces of advice on how to accomplish your goal of introducing some warmth to you system.
The easiest way to do this, (and probably the best), is that you could add tubes to either your preamp, power amp or source, or any combination of those three pieces of equipment. (The easiest being to add tubes to the preamp, as preamp tubes tend to be cheaper and last much longer than power amp tubes.)
However, if I were in your shoes, and since you have the Ayre pieces for both your preamp and power amp, and since they do have a synergy together, I think I'd opt for putting tubes in your source unit. The Audio Aero Capitole II SE cdp is in somewhat the same price category as your Linn Unidisk 1.1. (The prices for them used are about the same, so you should be able to sell one and buy the other for the same price.) The Capitole II SE has a nice touch of warmth to the sound, at the expensive of giving up a very small touch of resolution, IMHO anyway. (I like the Capitole II myself, and would probably go for it over my Resolution Audio Opus 21, but I'll be honest and state that it was only slightly better than the Opus 21, and I did not like the top loading aspect of the Capitole II, nor did I like having to use the puck. But, I am an analog fan first and foremost, and so I only use my cdp occasionally, so it does not matter all that much to me which unit I have, and since the Opus 21 is cheaper than the Capitole II, it made sense for me to not upgrade. However, from an absolute sonic standpoint, it is difficult to fault the Capitole II SE.)
So, I would recommend that you buy one, and compare it to your Linn to see which you prefer. (And if it were me, I would listen to the Capitole II for at least a solid week or more, and then swap back to the Linn. At that point, you will then have a good idea of the sound of each unit over the long term. A/B swapping can sometimes lead you to the wrong conclusion, as you'll sometimes pick one over the other just because it is new and different, rather than actually better.)
One more word of advice: Don't try to swap out cables to make the sound warmer. That is a band-aid approach that leads down the wrong road, IMHO.
The easiest way to do this, (and probably the best), is that you could add tubes to either your preamp, power amp or source, or any combination of those three pieces of equipment. (The easiest being to add tubes to the preamp, as preamp tubes tend to be cheaper and last much longer than power amp tubes.)
However, if I were in your shoes, and since you have the Ayre pieces for both your preamp and power amp, and since they do have a synergy together, I think I'd opt for putting tubes in your source unit. The Audio Aero Capitole II SE cdp is in somewhat the same price category as your Linn Unidisk 1.1. (The prices for them used are about the same, so you should be able to sell one and buy the other for the same price.) The Capitole II SE has a nice touch of warmth to the sound, at the expensive of giving up a very small touch of resolution, IMHO anyway. (I like the Capitole II myself, and would probably go for it over my Resolution Audio Opus 21, but I'll be honest and state that it was only slightly better than the Opus 21, and I did not like the top loading aspect of the Capitole II, nor did I like having to use the puck. But, I am an analog fan first and foremost, and so I only use my cdp occasionally, so it does not matter all that much to me which unit I have, and since the Opus 21 is cheaper than the Capitole II, it made sense for me to not upgrade. However, from an absolute sonic standpoint, it is difficult to fault the Capitole II SE.)
So, I would recommend that you buy one, and compare it to your Linn to see which you prefer. (And if it were me, I would listen to the Capitole II for at least a solid week or more, and then swap back to the Linn. At that point, you will then have a good idea of the sound of each unit over the long term. A/B swapping can sometimes lead you to the wrong conclusion, as you'll sometimes pick one over the other just because it is new and different, rather than actually better.)
One more word of advice: Don't try to swap out cables to make the sound warmer. That is a band-aid approach that leads down the wrong road, IMHO.