AD, I don't know why you suspect that you may be "missing" something; what you report is fairly typical. That 98dB peak uses 10 watts or a bit less for the M80s. The overall volume attained at specific volume level settings varies with the strength of the input from the source materials. Pre-amps typically have a gain(fixed, as is the amplifier gain)on the order of 15dB.
And right you are Sir; thinking about it a little bit, I remember sometime ago I did a little test. I plugged in my old Carver C2 preamp (which I now use as a phono amp only via my prepro) directly into the amps. The volume control knob has a scale from 1 to 10; at 3, it was MUCH louder then then half volume on the prepro. Needless to say, so was the noise.
So I dug up the specs on the Carver: MM Gain is 35 dB, while I suspect my prepro is in the range you suggested: around 15 dB.