What makes Telefunken tubes sound better?

I've reached the point in sampling 12AU7 tubes below $100 per tube where I'm having to buy tube crates to keep the pairs I've sampled. I've tried NOS and new, but out of all I end up back with Telefunkens as having the least distortion and fastest transient response. Yet the Telefunken internals are not the best materials/quality; NOS Mullard CV4003 or the new Gold Lions have higher build quality for materials and precision.

Are there links somewhere that talk to what is different for the internal design/construction of a Telefunken tube? I'd like to support newer tube manufacturers based on educated consumerism, and hope that we can get someone to replace Telefunken at an affordable cost before NOS stock is no longer an option.