For your requirement of a Band pass filter please refer to the thread with subject "BP filter" . There were some excellent links to sites on designing filters - should answer all your doubts on filters. Locate the posting by Michael J Karas.

And as to the unreliable PNP/NPN pairs, I am sure you are exceeding the power ratings at some point - maybe the starting or reversals. Otherwise why should they fail ? But if I were you, I would use a MOSFET - thier ratings are far higher and they have negligible ON resistances. Try them - I normally use them for 24VDC / 2.2 Amp PWM drivers for DC motors. No failures.

Raghu