marshall jtm-45 reissue questions

Y`all- I just played through two seemingly identical used JTM-45 reissue heads; one with an `X..... ` serial #, the other with `Y..... `. They sounded considerably different: The `X` sounded less trebly and `older` (sweeter and more compressed, mayhaps?), and the `Y` had more highs and sounded more `modern`. The `Y` also seemed louder with a bit more gain.
What might be the internal differences between the 2? They both sounded good, but I`m leaning slightly toward the `X`.

It`s not possible to know, because any number of things could have happened to either one of those amps.
Even things as simple as changing tubes, and which tubes were selected, and how hot or cold the amp`s output tubes were biased.
JTM45 amps also came with a variety of different output tubes as I recall. I think that some came with KT66 tubes, and some came with those wretched 5881 tubes, and I bet a bunch came with 6L6 too.
Modding JTM45 amps isn`t unheard of either.

I popped the back of both the `X` and `Y` JTM 45 reissues. The `X` had Svetlana 6L6s and 1 tall blue filter cap and 1 short black filter cap near the GZ34 rectifier. The `Y` had Sovtek 5881s and 2 tall blue filter caps.
After playing through both again, I decided on the `X`.