I covered the vents with blu-tak and bass response increased by tenfold, as expected. still detailed and doesn't muddy the mids as far as i can tell. and it lessens the upper midrange and treble too!
funny thing is when i crank up the volume starting from 1/3 to max, the bass stays the same except that it gets a bit distorted when the volume is at 4/5 to max. and when the volume is at 3/4 to max, the treble comes back again while maintaining its mids, and bass.
i like

used t51 as a source, no amp.