I have no experience cooling amps but lots in cooling PCs. Extra cooling is always good but I will add one small caution.
If you are thinking about modding the box to add a new fan(s) into the chassis to compliment the already existing fan, there is one small danger. Adding a more powerful fan can sometimes push or suck so much extra air through the existing fan that it ends up moving faster than it's motor wants to turn it and it can possibly burn it out. Some fans are sensitive to this while others, not as much.
You have to give a little thought to maintaining a balance of air flow in verses out when you go with multiple fans. Other than that, it can produce great results.
If you are thinking of adding fans externally to your shelving, then no problem. Find something that balances strong flow vs. noise. More air flow = more cooling but you don't want to introduce too much noise either. Again, a bit of research will get you a strong but quiet solution.
