Mcphile, the Klipsch really do prefer an amplifier with a high impedance output, if you want detail, smoothness, extension and impact all at the same time. Tubes are usually the answer for this.
If you just can't handle that, try putting a high power 4-5 ohm resistor in series with the transistor amplifier. That will simulate a high output impedance. A fair amount of power will be absorbed by the resistor, but you should find that the speaker is sounding better otherwise.