I have tried 6L6's (too boomy and too much treble) and KT-66"s (very boomy but very hi-fidelity clarity) in my Marshall JTM45 and have gone back to the 5881's. These tubes have a very even response. They do not over do the bass or treble frequencies. They have a nice bite to them and excellent clean tone clarity. Slightly flat and not hi-fidelity per say, just very even and workable. They also soak up overdrive and distortion pedals very well. Not perfect, but definitely very good. Will buy again.