The output level of the M33 is lower for a given decibel reading than you’re used to. If you are getting adequate volume at -20 dB, you still have 20 dB of room, so it seems there’s nothing to be concerned about.
The decibel readings on a preamp are in a sense arbitrary, and often are related only to the unit’s maximum gain.
From an engineering point of view, usually it is considered better for signal-to-noise ratio to be running any volume control at a higher, rather than lower, level. It means less gain is being thrown away.
If it were me, I’d be more interested in the sound quality at my usual listening level than the knob position required to get to that level. The exception would be if you can’t get enough volume to suit you . . . then, there would be a problem.