I have heard many of the worlds "great" orchestras, and I've heard many of the lesser known orchestras. Trust me, the Chicago Symphony can play a concert poorly and without any enthusiasm at all, and the Des Moines Symphony can play a concert that will knock your socks off.
There are plenty of smaller, regional orchestras in the US that are just superb. They may not have the last word in finesse, brilliance, and power, but when the stars are aligned just right, some magical things happen. I've been terribly disappointed at time with expensive concerts by Los Angeles, San Francisco, Chicago, Cleveland (!), New York, Philadelphia, Boston, Minnesota, Dallas, Detroit...the list goes on. And I've been thrilled to death at concerts by San Diego, Phoenix, Tucson, Colorado, Des Moines, Grand Rapids, Erie (PA), Kansas City and many others. It's a combination of things: the music, the setting, the conductor, the audience and other intangibles. Many of these smaller orchestras put on shows every bit as good as the big boys.
It's easy to see why there are so many good orchestras: the music schools just keep cranking out highly trained professionals at the rate of about 30,000 per year. These people want to play somewhere, and so even the hinterlands have top notch players available. 60 years ago the Big 5 were something special, but now there are dozens of orchestras that play as well. The only thing holding them back is a serious lack of conducting talent. The days of Ormandy, Szell, Toscanini, Walter, Klemperer, Cluytens, Boult, Barbirolli and countless others are long gone...