This is what I've been listening to in the last week:
I've seen Chicago in concert three times, once with Peter Cetera and twice without.
I honestly can't remember if anyone opened up for them the first time, right after Chicago 17 was released. It was 1985 and it was my first concert without my parents (my first concert was with my parents, Elton John's "Too Low for Zero" concert tour). The date of the concert is indelibly printed in my mind, as it is also the date I got my braces on - April 16, 1985.
The second time I saw Chicago, it was at the Oregon State Fair. They were on a double bill with Hall and Oates, and my husband and I had 4th row side section tickets. Unfortunately, they were also 5 feet in front of speakers... :) My husband will tell you I have HORRIBLE speaker karma...doesn't matter how good the seats are, the speakers will be right there (either above us or in front of us, or line of sight). The concert was still fabulous though, even though there was no Peter Cetera. I didn't know what to expect before seeing them, so it was all good.
The third time I saw Chicago, it was at the Clark County Amphitheatre in Vancouver, Washington. Fabulous show, lousy location. Horrible parking, sucky entrance policies (no umbrellas allowed but they only *ask* you whether you have a weapon), extremely expensive food and drink, plus some heavily intoxicated folks sitting right in front of us.