They make XLR to TRS cables. In fact I have one for my DI, bought it at guitar center.
The whole terminology is fucked in this thread, but to simplify, TRS can be used for both unbalanced stereo, or balanced mono. A fun fact is that XLR only works on balanced/unbalanced mono, unless we are talking about AES/EBU digital audio which we are not. The if a TRS cable is connected to an XLR on the other side, that means that the cable is designated for balanced mono applications.
you could use a TS to XLR for the monitors, but as mentioned, you will have a 6db loss in volume, this is because both the cold (-) and ground connections will be tied together.
The difference between balanced and unbalanced. Balanced has a separate ground, and cold (-) terminal. The ground is used to reference the +/- connections, unbalanced TS just has a hot (+) and a ground. A TRS for stereo uses both the tip and ring as a hot (+) and both grounds are tied to the sleeve.