The Biggest Risk: Signal Level Clipping
The biggest risk in adding another device in your signal chain is signal level clipping.
Do not run line-level signals into microphone-level inputs. A DJ mixer can and will clip a microphone input on a mixing board.
More Gain Is Not More Volume
Do not use a secondary mixer to increase signal gain going into your speakers. The maximum output of your DJ controller/mixer is enough to get the speakers to their maximum output capacity. If the system isn't loud enough with the DJ controller turned all the way up, you need more system — not more signal gain.
Don't Trust Your Eyes — Trust Your Ears
Your DJ mixer's outputs are very unlikely to clip. It's much more likely to clip their inputs, and even more likely to clip the inputs of another mixer. Turn up your DJ mixer until the speakers are as loud as you want them to be, regardless of the indicated level.
Why Signal-Level Clipping Is More Dangerous Than Amp-Level Clipping
Amp-level clipping can be prevented in the internal processing. Signal-level clipping can't be prevented in the amplifier — so the amplifier amplifies the clipped signal. This can damage drivers in the same way as amp-level clipping, but the limiters can't prevent it.
The Best Way to Add Mics to a DJ System
Mix microphone signals before going into the DJ mixer and add them to one of the inputs on the DJ board. Some mic mixers have mic-level outputs that can connect directly to the DJ mixer's mic inputs.
Conclusion
If you run an additional mixer between your DJ mixer and speakers, take great care not to clip the inputs to that mixer. The speakers have a maximum output level that is unchangeable regardless of how much signal is sent to them.
