This Tube Amp Doctor 6L6GCM-STR Redbase pack wraps up a full matched quartet of Selected Premium valves designed for souping up the power amp section of tube-driven guitar amplifiers. These valves are already renowned for their ultra-clear and transparent trebles, powerfully defined bass response and warm and sweet overall sound, and the direct dynamic response of these 6L6 classics is also incredibly impressive. So you're assured of consistently reliable performance, every one of these valves has undergone a strict quality control procedure before leaving the factory in Germany.