I have put these in every one of my Marshall amps (except the Handwired 1974X) and they really bring out that "Marshall" tone. I've even used them in some Hughes & Kettner and Crate tube amps. They've made every amp in which I've placed them sound WAY better. Unless you're re-tubing a TriAmp MK II the $60-$100 to re-do the preamp
tubes in a standard 100w head is well worth it.