One thing I noticed about the suggestions in this thread is that you are using a range of 70 db and push the needle up to +5 to get 75 db (if I understand correctly). I have a very similar SPL meter (different brand but looks exactly the same) and the manual says that if the needle goes above ~+2 or 3 you should switch to a higher range. Reason being is that if you do the battery test its basically pushing the needle as high as it can go with full voltage applied, and if your batteries are low (say +2 during batt test) then there's no way to get a consistent reading at +5. It would only ever get there due to fluctions in the bass frequencies that cause the needle to 'swing' past the +2 up to the +5 for a short time before it drops back down. This could explain why your sub needs to be so loud, to get lots of these fluctuations to keep the needle close to the +5 position. If this is the case, I'm not sure why the other speakers are able to be calibrated at +5, possibly because they use higher frequencies and the fluctuations happen much more often.
My suggestions is to calibrate at a different level, such as 70 or 80 db, to overcome this limitation of the analog device. This way you will be able to see fluctuations both above and below the reference point and will not be depending on the battery level.
