Costas,
The "journey" with the RKV is not frustrating at all. So far there are only like three tube options for it: RCA, Polamp, and Telefunken. None of these tubes changes the overall character of the sound that the RKV produces. The only change I can detect, so far, when I swapped tubes is in the detail department. Everything else seems to still be there from before, but now there is just more to listen to than before the swap.
I think some basic tuberolling info might help you here. For tube amps, the vast majority of the signal amplification is done thru the tubes, other than hybrids, so when you change tubes you can change the sound characteristics and signature totally when you swap out a tube. Each manufacturer has slightly different design changes, different materials, different construction, etc that make the sound slightly different from one tube to the next. Audio Valve designed their amp around the Polamp tube, and that's why it changes so little when you tuberoll it. The audio signal only runs thru the tubes and nothing else in the amp, and it seems that the available tubes are very similar in design, so their sound is very similar. It's not like the 12ax7 series where there are some 40+ different combinations of manufacturers and configurations that can be rolled into the same spot. (5751, 12au7,12ax7,ECC803S, etc, ad nausium)
Other tube amps, like the Melos, MG Head, Reality, et al, changed drastically when the tubes were swapped. It seemed like you had to give up something to gain something else. If you wanted great bass you chose one tube, when you wanted great detail you used another tube, etc. And when you did so you gave up something that you wanted, I got more bass, but lost the warmth; I got more detail, but where's the bass, etc. This is a generalization I know, but you get the general idea.
And yes, the journey to find EXACTLY what you want in a tube amp can be frustrating
and expensive. The easiest way to find out what combo works/doesn't work is to ask around here first and see what other people say. Research before buying is the name of the game. Read the various reviews that people have posted about tube combinations. Most here can narrow your choice of tubes down to a couple for each tube spot in the amp you buy. Those who can afford to buy and sample are somewhat rare, but they are helpful in helping those of us who can't do so decide before buying.
I suspect that there are no tone controls on headphone amps as the vast majority of users would throw a fit, claiming that it will affect the sound quality. It would to some degree, and most are purists who want the best signal possible coming thru without adding extra junk to boost some bass. You'd be better off buying a set of cans that had more bass and leave the audio signal as pure as possible.
Hope this helps answer some questions, and not add too much confusion!
