the thing with better sound is that you don't know it until you hear it. I don't know how many times I've felt that how can things possibly sound any better than this, or now it doesn't need to get any better etc. and then you try something else and forget all about it. someone once told me to not spend time and money on "sidegrading", when you want to upgrade you want UP, several steps! that could mean spending more money, but way too often I see people comparing very similar things in the same price range and maybe that's why the difference is not bigger. also, every system is different, some are more revealing and transparent than others. a "problem" with discussing things online is that we all have different angles and perspectives.
there's absolutely a lot of knowledgeable people out there that we need to listen to, talking to a serious tube dealer can be very interesting or even with people who have been rolling tubes since before I was born, it's not like I'm coming up with new tubes myself and I don't consider myself an expert in any way. tubes is not a new thing, and many of the best ones are over 80 years old and not something that was invented last week.
I'm not really interested in new tubes, old vintage tubes is in general the way to go, there might be a few exceptions, but when it comes to tubes older is usually better. that also applies for specific tube models, an RCA tube is not the same as just another RCA tube, an RCA built in 1950 is usually more interesting to me than if it was built in 1960. it's not only about the years of course, the specs for tubes are very different and they produce different results, the tricky part is to figure out which one you want, and then how to get it.
although, even if people are skilled and knowledgable and have a lot of experience, a lot of people are unfortunately sometimes stuck with old truths and have never really explored anything outside of the regular path. just as an example when I tried e80cc instead of ecc82/12au7 I wrote about it on a local forum and one of the most experienced and respected tube guys I know actually called me up and warned me about this tube and told me to be very careful. he really wanted to advise me to NOT use that tube. but confirming with Lampi it turns out it's perfectly fine to use both e80cc and 12bh7 instead of ecc82/12au7 in Lampis. today they even ship B4 with 12bh7. for a reason. it's not like B4 is a completely new design than B3, they only decided to change supplied tubes to something better. if you just do a quick google you'll find a lot of testimonials about people preferring these tubes instead of ecc82/12au7, it's not something that someone came up with yesterday. old knowledge, but it still needs to be brought back into the light every now and then.
having an open mind and being curious to explore and do your research will reward you with a better performing DAC. but it's also perfectly fine to just stick to the standard tubes and don't worry about it.