Your recourse is against the DJ group which attacked you, and since you know who they are, you can do either or both of filing a police report and pressing charges, or else suing them for the value of your equipment. As the ones who attacked you, they are liable.The club or hiring organization are most likely not liable for you loss. One person (or organization, business, etc.) is not liable for the criminal actions of another person or group of persons except in very exceptional circumstances. You would have to sue them and show that the security was wholly unreasonable for a venue and activity/event such as they were having, and that could be difficult to do: for example, if they were dealing with another situation at the time, they did nothing wrong--there's a limit on what any reasonable level or amount of security can do at once; or if it looked like it was safer for their customers to let the other group exit than start a potentially large fight or melee, then they did the right thing. Showing security acted unreasonably may be difficult--sue (or press charges against) the attackers instead.