if you have an older sub with coil on plug set up and its idling a bit rough andf lost a bit of its pep have a think about replacing the coils. i got some on E-bay a few weeks ago for 140$ and it has blowen me away how much it has improved the car, even dropping a couple of pounds boost it pulls harder, idles smoother and sounds crisp.
my car is a 1999 b4 and has done 200.000 ks so the packs must have sparked a trillion times and been cooking on the motor for 17 years, its my third motor but all the ignition stuff got swapped over to the new motors.
they are still on E-bay for 140$ down from 1000$, the best 140$ spent on the car ever.