Kathleen,
We just got back on the 13th so I just missed your window. My caution on classical concerts in Prague is that it all depends on your expectations. I regularly attend the symphony here in Portland and make any great orchestra a priority (subject to difficulties that you describe) anywhere I visit. I was fortunate to book tickets to see the Prague Symphony well in advance and saw an excellent concert of Wagner and Dvorak. The conductor was in complete command of a world class orchestra at the Smetana Hall.
There are a number of opportunities apparent in walking the Old Town to see other concerts, usually about 60 minutes and featuring a number of familiar selections. The Four Seasons of Vivaldi, The Moldau, and Dvorak selections, (often the Largo from the 9th symphony) and Bach are usually featured. One program I looked at closely had 13 pieces to perform, two of which, The Moldau and the Largo, would have taken 1/2 of the program if not cut. To me at least, this (and many others) looked like appeals to the casual tourist who wants to see something musical in Prague. My guess is that these are usually fine musicians performing in attractive spaces but not the programs I look for. So, if an hour of familiar classical music in Prague is what you are looking for, I think you can find it, and I'd be surprised if it is not well played. It's a very musical place. If you want to see a fully rehearsed season concert by the Prague Symphony or the Czech Phil, you probably need to plan ahead.
