Very good advice above, particularly from Kiddman nad Grannyring. Given that the speaker is something of an unknown element, it is a prime suspect. Also, different model speakers vary much more in sonic characteristic than do any electronic component. You should check it out by making direct comparisons with other speakers. If the problem IS with the speaker you should really consider dealing with the problem by either replacing the speaker or modifying the speaker before considering chasing compensating with other component choices.
Any extreme effort to compensate for a fundamental imbalance of flaw in one component by choice of other components will be a big waste of money and is not likely to work. Components may subtlely complement each other, but, they cannot correct for weaknesses of others.
It is understandable that you have a fondness for the speaker you constructed. That is why Grannyring's suggested crossover upgrade is a good first attempt at curing a problem if it turns out that the speaker is the primary source of the problem. This is a not too terribly expensive approach and you can go back to the original components if the modification makes matters worse.
Before doing anything at all that involves spending money, try changing speaker placement or moving the listening chair. Tonal balance can be changed quite radically by placement changes and there are infinite choices one can experiment with. With most speakers, the easiest way to change upper frequency balance is to alter toe-in of the speaker and the backwards tilt of the speaker. You could also try some simple room treatments to tame excess reflection of higher frequencies (e.g., put up tapestries on the walls).
Good luck.
