How do I know what type of jack is on the amp? It is an old Peavey.
You can check
here, or you can wait until someone responds...who knows, they might have designed the thing.
My real question is that I want to send the same mix to both the amplifier and to a personal powered keyboard monitor...
stagebox TRS >>> TRS splitter >>> One side, TRS cable to input on powered monitor >>> other side to input on amp.
OR
Stagebox TRS >>> amp . . . .Then Stagebox parallel XLR >>> Powered monitor
From the sound of it, either should work. Most modern amplifiers have their own parallel outputs that you could use, but your amp may not have them. Once you find out what the model is, you can determine that.
General question:
1) What happens when you plug a 1/4" mono TS plug into a balance mono TRS jack? Does it get the same signal just no longer balanced?
Well, the Ring and Shield (Pins 3 and 1) will be shorted together. This may or may not be a problem; if the input is capable able to adapt to both balanced AND unbalanced cable types (even though it's the circuitry that determines if a circuit is balanced), you would probably just lose 6dB of gain and the robustness of the balanced connection. If it's a short enough cable run and there aren't a lot of other cables lying next to, it''s probably fine.
Have you had any problems because of the current setup?