bass lower in vlc player

Have the neighbours actually complained, or are you just trying to avoid a complaint and you think that the bass frequencies will be what they are likely to hear the most?

What type of accommodation do you live in?

If the neighbours are on the other side of a shared wall (duplex / semi-detached / apartment), then sounds will be transmitted in other ways than just airborne waves, eg. speakers mounted on a hard stand on a hard floor will convey vibration through the floor and into the shared wall.  Mount the speaker stands on something padded to absorb the vibrations.

Everything has a fundamental frequency, which is the frequency that it will oscillate at.  This is best explained by trying to jump up and down on a springboard or trampoline too fast or too slow.  Each of these objects bends and vibrates at a particular frequency.  Hold a plastic ruler over the edge of a table and "ping" it with your thumb.  It vibrates at the fundamental frequency for the bit that is poking off the table.  Shorten and lengthen that vibrating length and the frequency goes up and down.

Long suspension bridges can begin to oscillate very badly in an arc if they are badly designed and are vibrated by something steady like a wind or lots of people walking in time over them.  A guitar string will begin to vibrate if you generate a sound in the room that matches the fundamental frequency or octaves of that.  The same is true of any room.  There are certain frequencies that are boosted falsely and cause horrible oscillation in a room if it doesn't contain sufficient material to absorb and dampen those frequencies.

Lots of soft things in a room (people and soft furnishings) tend to absorb high frequencies the most, whereas hard surfaces reflect the sound and you get a lot of reverberation that is mostly of high frequencies.  High frequency sound waves (treble) travel in very straight lines and are therefore bounced back off hard surfaces and absorbed by soft surfaces more easily than low frequency sound waves (bass) which has a longer wave (just like the shape of waves in the sea) and which can bend around corners easier.

That is the reason you hear boom, boom, boom when somebody like Merete drives past in her jazzed up car playing Trance music up loud.  The bass frequencies travel further and escape from the car much easier.  By the time the sound waves from a rock concert several miles away reach your house, the high frequency waves have been bounced around and reflected off into space, while the long persistent bass frequencies travel very much longer and all you hear is muffled boom, boom, boom.

Ask any environmental noise investigator or police officer what noise is complained about the most, and they will tell you it is that booming low frequency noise, whether it be neighbours playing music or industrial noise.  It is just the nature of sound, and it's impossible to stop it behaving that way.

When you see a tone control marked like this:

-5  -4  -3  -2  -1  0  +1  +2  +3  +4  +5

or the same on a circular rotary control the minus settings actively cut what they are set to cut, and the plus settings actively boost what they are set to boost.  A simple - 0 + treble control will be set to a wide range of high and mid frequencies and the simple - 0 + bass control will be set to a wide range of mid and low frequencies.  A graphic equaliser divides those wide ranges up into much narrower bands so it can cut or boost more specific frequencies.  By boosting treble it isn't cutting the bass, it is just making the music louder overall and making it more "tinny" because the treble is much more dominant than normal.  You can cut the bass frequencies, but the music will usually sound terrible.

As Merete showed you, with everything set to zero (a "flat response") on a graphic equaliser, neither the bass nor the treble are boosted.  If the music is still booming you can then concentrate on your speakers' tone settings.  Set them to zero if they are the sweeping - 0 + type for bass and treble, then eliminate any enhanced effects in the soundcard's software settings.

If you now listen to your music at a healthy and respectable volume level and the bass frequencies are still causing concern or annoyance, you really need to invest in headphones or sound insulation, starting with any surfaces that might possibly be vibrating in sympathy (that fundamental frequency thing again) with certain types of music.

Many subdivided offices use padding in between walls made from drywall sheets on a framework, or metal panels on a frame. The padding helps to prevent the transmission of sound through one solid surface between the rooms.  Recording studios use "egg box" foam rubber sheets and angled baffles to absorb sound that would otherwise be reflected back into the room and cause a mish-mash of echoes, but it also helps to soak up sound and stop it passing through the walls into the next door studio.

Experiment by angling your speakers as well.  It could be that the way they are placed doesn't cause you to hear an uncomfortably high bass content in that room, but may be causing an unnaturally high level of bass on the other side of the wall between houses.

Remember that in most types of houses that are conjoined in some way there will be shared structures in the form of wooden joists for the floors, shared roofing joists, and shared brick, concrete, or metal supports.  All of these can conduct sound in the form of transferred vibrations, and can then be exaggerated on the other side.

Walls and windows can make very good speakers if you stick a small speaker right onto them, and in fact this principle has been used in a number of applications like these:
http://www.feonic.com/
http://www.prodisplay.com/sound-pod-glass-speaker.html
http://www.multipanel.co.uk/soundPanel/soundpanel.asp

So, speaker stands on a hard floor could easily be causing vibration through the boards and joists and then using the wall between you and the neighbours as a large sounding device.  You would know this if you were able to stand in the neighbour's house and put your hand on the walls and the ceiling while your music was playing in your own home.  You obviously can't start ripping up floorboards and replacing hard connections with rubber dampers, but you should be aware of vibrations through solid media while considering how to please your neighbours.

There is no easy answer to your (or should I say your neighbours') problem short of exercising common sense and courtesy and trying to eliminate the kinds of factors I have mentioned.  Wait until they go out and then crank up the volume - and bass ;-)

Merete has to live in the middle of nowhere and has to drive half an hour to see another person because I'm sure only a hearing impaired lunatic would choose to live next door to her "trance" and boom, boom, boom club type music :-)