As a Chicagoan I have a lot to say on this subject, and I hardly know where to start. I attended CSO concerts sporadically through the '90s but have listened regularly in the last decade.
Before the 1966 renovation, the seats were not padded. Years ago someone told me that before '66, the musicians had trouble hearing each other. That was improved with the remodel, but introduced complications for recording as the sound of the empty hall changed considerably, less so for live concerts. I have no idea if that's true.
The next big renovation occurred in the late '90s (not '80s as previously suggested). An acoustic reflector made of curved plastic panels now dangles over the stage. This has proved to be a double-edged sword: The sound toward the rear of the hall is now brighter where it was previously foggy, but to my ears it is quite unnatural. The reflector acts as a giant microphone, and when the orchestra plays pianissimo, what reaches the audience goes no lower than mezzo forte. Woodwinds in particular sound exaggeratedly loud and even scratchy, while string tone remains dull and muffled. Toward the front of the hall, the brass sections sound rough, but this is not as bad if you're sitting farther back.
Since the second renovation coincided with the collapse of commercial studio recordings, few if any recordings have been made since then without an audience. I find the CSO Resound issues that I've heard, such as M3 and M6, overly bright an lacking in a natural perspective.
RCA engineers didn't seem to be happy with the results of the 1966 renovation. Checking the Martinon CDs of Ravel and Bartok/Hindemith/Varese, only the second suite of Daphnis and Chloe was recorded in Orchestra Hall, in 1964. Everything else was recorded at Medinah Temple. The Nielsen disc does not identify the recording venue.
I heard one concert at Medinah Temple with an excellent community orchestra, and found the acoustics not only superb but vastly superior to anything I've heard at Orchestra Hall. It is sad that only the facade remains, and even more melancholy to consider that circuses enjoyed better sound in this town that one of the greatest orchestras on earth.
Most good halls are shoeboxes. Orchestra Hall is unusual in that it is egg shaped. I think it's obvious by now that this venue is acoustically hopeless. My private theory is that it's just too small. An orchestra that can play as loudly as the CSO just needs more cubic feet of air. If only they could play just down the street in Louis Sullivan's Auditorium Theater.
Solti recorded M7 at the Foellinger Great Hall at the Krannert Center in Urbana, to my ears a good-sounding recording of a performance that's well above average for Solti, at least until the wheels fall off the bus in the finale. I have heard the CSO as well as U of I ensembles there; it is a terrific hall.
Eventually, engineers came to terms with the post-'66 Orchestra Hall. I have a strong antipathy to Daniel Barenboim's conducting, but his Alpine Symphony is a good (if clininal and uninvolved) performance captured in fine sound. I'd even say that if you really want to hear what the CSO sounds like, this is as good an example as any.
Having said all those nasty things about Orchestra Hall, I'll cheerfully point out the that the conductor really matters. I heard a horrible M9 with Barenboim; in the tuttis, the sound was a grey industrial sludge. But with Haitink on the podium, the orchestra sounds clear and transparent.
An intriguing prospect would be that the Harris Theater in Millennium Park could have been made larger and might have been a spectacular new home for the CSO. The acoustics are superb for ensembles up to medium size.
OK, I've poured out my soul. Anybody care to comment on the acoustics at Severance Hall in Cleveland or Symphony Hall in Boston?