Your understanding is way off. Loudness, as measured in dB SPL (Sound Pressure Level) and our subjective perception of what constitutes "somewhat louder," "clearly louder," and "twice as loud" is complicated and is not calculated by simply multiplying the decibels arithmetically.
A 3-dB increase in loudness most listeners describe as "somewhat (or a bit) louder"; a 6-dB increase as "clearly louder"; and roughly a 10-dB increase as "twice as loud".
Wall-mounting the M22s increases their lower bass output below 100 Hz by as much as 9 dB, according to Ian's measurements. That is heard subjectively (and I heard it in the blind tests) as the lower bass in the 40 Hz to 80 Hz range sounding about twice as loud, especially in the lower octaves where the M22s on stands normally roll off (gradually decline in output).
The decibel is a source of great confusion because everyone at first assumes there is an arithmetic relationship between power in watts and a speaker's sound output in decibels. I've written about this at greater length in the Axiom newsletter archives.
Axiom Resident Expert