May be the center speaker/channel has a ton of room gain, and your other speakers have lower sensitivity, even lower than their specified numbers. If using RCA can bring the trim back to above -12 dB then you are all set. RCA's voltage is half that of the XLR's so that's 6 dB right there.. Such a trick obviously works on the M28, but it wouldn't work on other amps such as Marantz's multichannel class AB amps because Marantz would drop the gain by 6 dB when you use XLR inputs, not sure about the Amp10 though.From Paradigm's site
Sensitivity Room / Anechoic 94dB / 91dB
That's what I did is use a RCA cable for the center channel only. But before its level is set, it's clearly louder than any speaker in my setup.