Add a welcome from me. I'm also 70+, audio and classical — though not aerospace; my background is film, mainly writing them. Yet Physics and Astrophysics always fascinated me, and Engineering too — all of them, and of course Music, have Mathematics at their core. I spent 9 weeks crossing an ocean once on a sailboat, and choosing books to bring was crucial. Moby Dick of course — where better than the middle of the South Atlantic to read it — yet six of the books, thick ones, were math and physics. (My avatar, BTW, was me back then, pointing to an engine part I had to make at sea, a bit of engineering of my own.)
I'm immersed in the middle quartets now, Op 59 — many overlook them. #2's Adagio is ravishing. Quartetto Italiano has the beautiful tone to express it. But so many groups have much to say. I have a few "cycles" — Italiano, Vegh, Talich, Lindsay — and I cherish them all because each excels in this movement or that moment. The Schubert String Quintet is exquisite, one of his final four works. Stravinsky called the Adagio "the most beautiful music ever written" and I can't suggest an alternative.
Chamber is my favorite classical genre now, maybe because my home system can make it "real", portray the scale, body and tonal richness of a live performance. I've never had a system, no matter how large and powerful, that could render a full symphony orchestra realistically — though concertos can do it for me, maybe because I focus on the solo instrument and the orchestra is less important.
I also use separates, and increasingly CD. Their quality has increased so much over the years, and getting a good DAC five years ago (finally) was another quantum leap.
I don't want to talk your ear off, so I'll sign off now. Hope to see more of you here.