Sometimes the best way to learn is to just buy the best (or worst or just cheapest) tube amp you can find and see if you can figure out its contribution (or distortion) to the music you like. Then spend more money on different tubes seeing if you can hear a difference between them. You can try to be an electrical engineer and decide, based on facts and theory, ahead of time, how things should sound. Or you can just go straight to the "play" phase. I suggest you do. Just understand that planers are less reactive to output impedance than dynamic headphones. They may not "color" as obviously via high-impedance outputs. A cheaper way to color the sound would be to just pick some random EQ curves and apply them. You won't get the even-order distortion of tube amps, but you will familiarize yourself with how different "shapes" sound. Recently I got tired of reading reviews of speakers and trying to understand measurement techniques since they vary. So I bought a $15 measurement mic, an Android App, and wasted hours of my life seeing how my measurements stood up to published data. It has been rewarding as I am coming to understand the process better, and interpret the results. To make a long story short, it isn't THAT hard to get the basic profile of a speaker nailed down. But any measurement I do will be a lot "noisier" than an anechoic chamber. Period. Turns out one hsa to be pretty creative even when they are trying to be "objective"!