It's great to hear that you're enjoying your A2 Balanced tube pre-amplifier from YS-Audio! The A2 Balanced pre-amp seems to have made a significant improvement in your music listening experience, particularly in terms of enhancing the holographic and natural sound quality.
It's not uncommon for tube pre-amplifiers to benefit from tube rolling, which is the process of experimenting with different tubes to find the ones that best suit your preferences. It appears that your decision to replace the stock Phillips WB5687 tubes with Amperex 7119 tubes made a noticeable difference, especially in terms of bringing back the missing mid highs and highs and enhancing the overall holographic sound. Additionally, upgrading the Chinese 12AU7 tubes to Amperex Bugle Boy tubes further improved the sound quality.
Investing in quality tubes can often enhance the performance of a tube-based audio system, and it seems that your investment in the Amperex tubes has been worthwhile. The total cost of $480 for the tubes, combined with the $1350 cost of the preamplifier, has resulted in a fantastic sounding setup for you.
Overall, it seems like you have found a winning combination with your A2 Balanced pre-amp and the chosen tubes. Enjoy your music listening experience, filled with Jazz, Vocals, Instrumental, and Classics!