This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
The best way to change the sound of your speakers is to change
where you put them. Ideally, the speakers should be located at
ear level, in front of you, squared off between you. It's then
a matter of fiddling with a) the angles, b) the distance apart,
c) the distance from you, and d) the distance from the wall.
Just moving the speakers around in the room or putting them onto
stands can make a major difference. For more on speaker
placement, see 13.1 below.
Other than that, speaker modifications can be a can of worms, or
can produce very subtle changes, which you might prefer. For
example, you might improve a speaker by adding some cross braces
of 1"x1" wood from left to right and from front to back. This
will stiffen the cabinet and reduce speaker cabinet wall
vibrations, which probably hurt sound quality. Alas, this will
be most effective with lower-cost and poorly built speakers.
Along similar lines, some claim success putting lead wire or
epoxy putty on thin parts of the speaker to damp out resonances.
You can try doing this to the thinner parts of the speaker
"basket" or frame, or to the front "baffle" or supporting panel.
Still another "tweak" is to add sound deadening felt pads to the
inside walls of the speaker. Instead of felt pads some advocate
sand-filled latex coatings on the inside walls of speakers.
Others advocate ceramic tiles held in place with "thinset".
Still others rave about commercial products like AC Glop,
Acoustic Magic, and Bostik Sheet. However, the people who rave
about these products tend to be the same people who sell them.
Any change along the lines of adding felt, cross-bracing, or
putty will have subtle effects on the sound.
For the brave at heart, you can replace old or cheap drivers
with better ones, but the results of this one change can be very
dissatisfying if you happen to get the wrong type of driver for
that application, and may never sound right, even if you use a
similar driver. Speaker system design is still somewhat of a
science and somewhat of an art. Throwing paint on a canvas
often makes a mess.
Whatever change you try, don't "burn your bridge" home. Be sure
that you can undo whatever change you did, just in case. Many
tweaks to good speakers, no matter how well thought through,
will correct for one flaw, but create others, or correct a flaw
that the designer had cleverly used to his advantage.