I've been to several concerts now by the TPO, the Thailand Philharmonic Orchestra. Their home auditorium is at Mahidol University, and it's a beautiful, small, great sounding theater. It's mostly Thai musicians, with a sprinkling of Japanese and Chinese, and a couple of Farang too. Maybe I'm not a connoisseur (maybe!), but I think these guys are first rate.
These concerts are a great bargain (I hope that's not too crass to mention). Three hundred Baht (ten bucks), one hundred for students, and every seat is front and center. This one happens to be a little "Top of the Pops"ish, but that would not work as a general assumption. They tackle modern, difficult and very serious stuff too, and what seems to be a complete catalog of your Beethovens and your Mozarts.
Yesterday I saw the National Symphony Orchestra, a different outfit. They were good too.