Several people have reported that tapewound strings don't work on piezo pickups on Carvin P-series basses (and presumeably other makes with similar piezo bridges), and some have conjectured that the fault is due to the tape reducing the pressure on the piezo pickups. See: here and: here and: here I have just determined the real reason: the ground connection for the pickups is broken due to the tape. It turns out the piezo pickups are grounded via the strings, and since tapewounds do not conduct, the ground connection for the piezo pickup is lost. The little metal piece at the bridge saddle needs to be grounded to complete the circuit. I solved this problem by wrapping the string with 1.5" of metal foil (copper adhesive tape used for shielding works great) between the point of contact with the pickup (the metal at the bridge saddle) and the point of contact with the bridge frame, where the string gets grounded. The tapewounds now sound great through the piezos. Happy playing!