The frequency spectrogram is different for each stage.
So the RIAA correction curve on the stages is a little different - not uncommon for phono stages and often responsible for differences.
Yes, it might explain the great bass you perceive from one of them
Take a look here.
Look at the bass end in particular, and one seems to have a little more going on at the extreme top end. There is a good 4-5db difference in some places.\