The important consideration is whether the rack is flush with the speakers, or behind them. IME, if the rack is at least a foot or more behind the plane of the baffles, there won’t be much if any detriment. The big exception is panel speakers, they're sensitive to any and everything.
Equipment Rack Between Speakers...Good or Bad
This question came up in another current thread and I thought it would be more appropriate to start a new thread to address it. My rack is between my speakers. In the past I have tried it off to the side and didn't notice any sonic advantage. I have seen in in the past that there are some strong feelings on this subject, and I am interested in hearing what everyone has to say.