just to emphasize what *some* of the previous posters said:
GET THE ROOM TREATED
- or maybe another room.
of course there's plenty of great monitors to choose from and plenty of funny techniques to shift bass frequencies up or down, but as long as you are not able to HEAR it properly, you're lost!
one advice: the Yamahas are active (admitted: not small

). nevertheless, try to carry them to an different place and set them up - in another room, best would be even in another building. an average living room with bookshelves and couches and so on is best suited.
take a laptop or a CD player and listen to some well mixed music - and your own stuff - there through the speakers. compare the sound you are hearing thru the monitors to the sound in some good headphones, in this very room!
do this for a while until you think you can recall the sound of a good mix.
back at your place again, start to mix the bass in the cans only, at moderate volume, without switching on the speakers yet. then add some of the other tracks and switch on the speakers, still at moderate level.
how does it sound? boring? well, then you 're probably on the right track. try not to change something in the LF range, just leave it like it is. bring the mix to an acceptable (rough) shape and take this mix to the car and listen...
why this effort? i made the - painful - experience that even headphones in an inapplicable room sound somehow unbalanced. obviously your ear (rather: the region of your brain that handles acoustic signals) strives to accept the actual room acoustics as "normal" and applies different criteria for "right" and "wrong".
just saw you're in Germany too... Hi!