Why is it that with today's technology we cannot make tubes sound better than the NOS of 1950s-60s????
I think that in the case of power tubes, they do. The current crop of EL-34, 6550, and KT-88's are very good. The Genelex KT-88 re-issue has gotten great reviews. In fact, a number of tube mavens with engineering backgrounds actually think the KT-88 re-issue is as good or better than the original.

As for the small signal tubes, do think they have had the same success. That's why I have a large stash of Amperex 6DJ8, 6922, 7308, along with 5687, 7044, and 7119 types for my equipment. To be fair, some of the Russian types (61NP, 12AX7LPS) are actually quite good.