The one thing I forgot to mention and its often a question is what happens to odd order harmonics when tubes are very closely matched. They actually increase, in fact they are summed.
If you go through the math of the transfer curves as a power series (I'll spare you that) even order components on each side of the O/T primary end up equal and opposite out of phase and will effectively cancel and the odd order components end up in phase, so will sum.
Controlling the generation of these different harmonics through all the different stages of an amp is like a recipe. As a builder you create your own recipe that you like and thats part of what gives a brand a signature tone. People obsess over specific brands of components and materials and there is some validity in that for sure, but generation and manipulation of harmonic content, gain and frequency response of every stage is the key to the front door, especially for an overdriven rock / metal amp. In any simulation software like LTSpice or TINA you can design a stage and do a fourier series to see exactly what the harmonic content of that stage will look like as well as a gain and freq response over its entire operating range. You have full control and analysis versus just tweaking out components and doing everything by ear. That part comes later to make any minor changes. This is what actual 'designing' looks like when done using a more traditional engineering approach. I've wandered off at a tangent.
Back to the question at hand, yes mix those tubes up as long as they are not wildly mismatched. You might be suprised with the results.
