An issue with using a mixer designed for high impedance unbalanced -10dBv connections is that it probably doesn't have enough gain to get the Mic level up to line level on most sources. The mic signal doesn't mind seeing the high impedance... but there's not much gain in a system designed to primarily mix -10dBv input signal and output a -10dBv signal.
Of course a trumpet, usually, does not need a lot of gain. :-)
Which is probably why it seems to work ok with the -10dBv input on the on board Sound line in described in the OP.
One thing to note about the Roland unit. The on board compressor is useful for (near)real time monitoring and sound tailoring. It happens well after the analog signal converts to digital so if you are over driving the analog stage and clipping the digital with overs the compressor is just going to make that seem louder.
The point being is that you are probably going to have to turn down the input gain on the Quad Capture mic input
a whole bunch until you find the sweet spot for the trumpet.
I recommend avoiding the digital compressor until well after you get the level settings figured out.
Singing in to a compressor while listening on headphones is a real treat... it effects your performance and helps you explore a relationship with the compression effect. I think the most compelling reason to use the compression on the recording track is to help a performer do this.
If you don't have an interest in that then it's easy to just wait till post to add the compression... which gives you the benefit of getting to try different settings without permanently committing until final mix down.
best regards,
mike