It's also possible that the bass was deliberately boosted on the mid-60s mono albums in order to compensate for the inadequate bass response of the cheap record players typically used to play them. This is my suspicion, but I've not been able to get confirmation of this from anyone who'd know for sure.  Certainly when you play them on a modern hi-fi system the bass tends to sound a little boomy rather than just naturally extended.

Further adjustments may perhaps have been made to the mono mixes (such as applying compression and brightening up the treble) for anticipated broadcast of the mono versions on AM radio. However, this was normally done only for single releases, and the adjustments would be made at the 45 rpm cutting stage.