All standard Axle-Back and Valve-Backs may be used on NPP equipped vettes without causing a CEL. Variable Sound NPP Valve exhausts (Part # 14777 and 14776) can only be used on vettes with NPP.

C7 Corvette Sound Level Guide: Double X-Pipe (Part # 14761) when paired with an Axle-Back or Valve-Back will increase the overall sound level of the Cat-Back system. *Variable Sound Axle-Back's with NPP Valves open are similar to Xtreme, valves closed are similar to Sport
Sport Axle-Back + X-Pipe = an Xtreme Cat-Back
Xtreme Axle-Back + X-Pipe = an Xtreme+ Cat-Back