Here is a very simple crossover modification that will substantially improve the performance of the popular Morel 403.5 speaker.  The mod has been auditioned and approved by Morel, and implementing it will not void the factory warranty.  The Morel is available at http://www.renaudio.com/homeaudio/mlp403.5.htm.

This model uses a dome midrange that has a huge 10 dB breakup mode between 6 kHz and 9 kHz.   It looks like this:

The factory crossover does not adjust for this, and the result is destructive interference between the midrange and tweeter in the mid treble, and ringing due to the midrange breakup.  Fortunately, the midrange peak can be suppressed by adding a simple trap filter composed of a 1 ohm resistor, an 8.2 uF capacitor, and a .06 mH inductor, all connected together in series and then wired between the terminals of the dome mid.  Just take the midrange out, remove a little bit of the insulation near the terminals, and twist the 3-component trap circuit to each section of bare wire.  It doesn't matter what order the components are in, or which end you connect to which midrange wire.  You should solder the new connection, but if that isn't possible, make sure the twisting is tight and cover with electrical tape.  This produces the following response curve:

I can send you the little .06 inductors -- they aren't commonly available.  The cost for a pair with shipping would be $5.  I can also send you the whole circuit ready to install.  That will cost you $20 per pair (my cost for the Mills resistors, poly caps, and inductors).  Just e-mail me.

The change in phase relationships between the mid and tweeter will require that the wiring be reversed on the tweeter.  So take the tweeter out and switch the connections on the back.  That's it.  The sound will now be much more focused and accurate.  If you have any questions or problems, just e-mail me.