Jasper, "grief" wasn't described, but as Alan pointed out, the theoretical objection is that the sound runs through two preamp sections rather than one, adding a small amount of extra distortion. This is unlikely to be audible, and if you like to use the PM16, carry on.

If the main speaker level trim was set in the 6004 to the minimum -12dB available(possibly indicating a lower setting was wanted)during calibration, lower the PM16 volume control a bit so that the calibration number comes out higher.

Edit: Roy, once the level is properly calibrated, the PM16 level control wouldn't have to be changed; any volume changes would be done in the 6004.
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Enjoy the music, not the equipment.