May I kindly add my point of view, please? When I owned the sound company, we made our own racks and they were very heavy on account of the fact that back then even solid state power amps had massive power transformers that required a lot of physical support. We had to go to extremes to properly buffer and secure these amplifiers so they would not be destroyed on the road. Instead, both my shoulder joints got destroyed moving that #$%$# around.
These days the situation has drastically changed on account of class D topology and switch mode power supplies. The individual components that comprise the new amplifiers are not all that heavy any more, so the size and weight of a power amp has been vastly reduced. By nature a steel chassis will always have some give to it, which is known as its tinsel strength. Were it not for this, the metal would crack under stress on account of being excessively brittle (i.e., iron Vs steel). This is what I believe accounts for the few millimeters of sag noticeable at the back of a modern day component that may happen to be mounted in a rack. It doesn’t necessarily mean anything is wrong.
QSC has a long standing reputation for manufacturing absolutely top quality gear, and it appears that they have as usual taken all factors into consideration when designing their new line of products. Given the characteristics of high quality steel and the actual weight of the components that go to make up one of their new amplifiers (which is relatively low compared to past products), there is every reason to believe that sufficient strength is present in the chassis so that properly securing the component to a properly made rack by way of its front panel will be more than strong enough to provide for adequate stiffness and security. There isn't anything all that heavy in a power amplifier anymore, and steel is very strong, actually.
If desired, it shouldn't be that hard to support the back of an amplifier in a rack with a thin piece of stiff rubber or wood in between the components or between the component and the floor of a rack for piece of mind’s sake. Just make sure its no more than ¼ to ½ inch wide so it doesn’t cover very much of the bottom or top of the component. Personally, I wouldn't worry about a few millimeters of sagging as long as the rack your using is strong and won't shatter or twist. That's where I would put my focus. That is, I would make sure any amp rack is strong and will not shatter or twist! In this case, if QSC doesn't make an optional accessory rear mounting bracket for the particular amplifier in question, then I wouldn't worry about it. If they do make such a bracket, then it should be pretty easy to just use that if you wish.
Get your new rig going so you can concentrate on your playing! Let us know what happens. Oh, wait! Did I mention to make sure the rack is strong and won’t shatter or twist? I would definitely do that. Good luck!
