Some may not know that parametric EQ can provide precise sub bass boost without affecting mid bass/mid range.
In Rockbox, you can use EQ's low shelf filter to boost sub bass tightly and precisely. An example low shelf (LS) filter setting is: 100 HZ as cut off frequency, 0.7 as Q (this band width factor should not be changed in LS), +3 db as gain. This setting will give a +3db boost to bass below 100Hz without affecting frequencies at 100Hz and up (i.e., this setting will not distort or muddle mid bass/mid).
But note that software bass gain of > +6db may distort, if your DAP/amp cannot supply 4 times more power there. Hardware bass gain (e.g., that from Fiio E11, E12 or E17) can supply more power and limit distortion, but in exchange you loose the precise frequency control of parametric EQ -- hardware bass boosts are not implemented the same way in every amp, for example, E11's level 1 bass boost is a lot tighter than that of E17 (E17's bass boost goes way beyond 100Hz and affects mid range, while E11 does not).