The unclear highs are probably due to a rolled off response from the tweeter, not a modified curve from the crossover. I doubt a new crossover will fix this. If I were you, I would invest in better speakers. however, if you are intent on "fixing" your old ones, you will first need data on the response of the individual drivers to properly integrate them with a crossover. If you have the measurement tools to do this, or have access to the original measurements from DBX, then you can go about designing a new crossover. I would personally recommend that you go the active route, using two Behringer DCX2496 units (one for each speaker) and enough amplification (5 channels per speaker).