NNck,
As I understand your setup, you need a tube linestage (instead of a buffer) because you need the volume control. Does the signal then get passed back and through the JC2 circuitry so that the HT passthrough is acting sort of like an external processor loop?
I think you could have some interesting fun trying different tube gear in such a loop, but, it is hard to predict the result with ANY given component and, while you might get some idea of what tube gear sounds like, the reasonable conclusions you could draw from that experience would be somewhat limited.
First, more than solid state gear, I think there is a HUGE difference between different brands/types of tube gear. My system is currently tube-based and I suppose I would be considered a tube person, but, MUCH of the high end gear I least like are tube-based. There is a truly striking difference in sound between, for example, the VTL/Audio Research/BAT sound and the sound of Audionote/Shindo/Kondo gear. There is nothing much in common between the sound of low-powered single-end triode tube amplifiers and a pushpull pentode amp using 6550 output tubes.
To add to the complication, compatibility is harder to achieve with tube gear. On paper, it is much harder to match a tube amp to a variety of speakers (power needs are harder to meet, low speaker impedance may not work well with high output impedance of many tube amps, etc.), it would seem to make sense to do what you are proposing, which is to introduce tubes at the line level. But, in my own experience, even when a tube linestage is supposedly electrically compatible with a particular solid state amp (i.e., low enough output impedance from the linestage), the actual practice is quite wildly unpredictable. So, it comes down to actually trying a combination--I would never try to predict the results. I have gotten more predictable results with solid state linestages feeding tube gear than the other way around. I am not saying tubes feeding solid state doesn't work, I am saying the results are unpredictable, and more often than not, you will not get "the best of both worlds" but something disappointing. If that happens, don't give up or draw any conclusions about tubes.
I do like your idea of at least trying tube gear in the chain. If by chance or by experimentation you hit upon a nice sounding combination that would be great. But, by no means can you then conclude you are enjoying what others consider the "tube" sound. It would be just one of many aspects of "tube" sound.
Be forewarned that it may then set you on a path to trying other tube components, and perhaps, trying different tubes in each tube component. Again, there are vast arrays of different sound that one can get by changing tubes.