I go back and forth between three tubes. Stock, a Phillips PCC88, and a Valvo E288CC. It depends on my mood and what I want to listen to. Music sounds different to me every day. Sometimes I just need a slightly different flavor. If there's one thing I've learned by bouncing between these tubes, it's that there's no one perfect tube. They're all great, but sometimes I just have to swap them. Part of the magic, I think, is novelty. This is definitely a component to the rolling obsession. Once you get too acclimated to the sound of one tube, you start wanting something different. Fortunately for me, I have a couple to rotate and that generally does the trick. I just stick with one until I forget what the other sound like... ...and then when I finally swap I have a new sound to explore. I think a couple of cheap tubes in addition to the stock should be sufficient for most. I mean, you can keep hunting for the holy grail tube if you want, but I don't think it exists. I tend to leave it at, "I really like this tube." or "This tube sounds great today!" I think that's really the beauty of rolling tubes. Once you pick a couple of tubes that you know are good, you have a way to easily change the sound of your system whenever you get ADHD or perhaps the onset of shinyitis. Think you need a new amp? Maybe all you really need to do is grab that other $20 tube off of the shelf and just be happy! Way cheaper than swapping between dacs, amps, or headphones! But as it is in all things, it's a matter of degrees.