I use two of these little gems in a Primaluna Prologue Classic integrated amp along with two Philip`s miniwatt 12ax7 and four Svetlana Wing =C= EL34`s. The synergy among-st it all works really well giving me a rich warm well balanced tone. Mullard NOS valves are really worth paying the extra, they are exceptional. I did find for my taste having four Mullard`s ( 2x 12ax7 & 2x 12au7 ) in the Preamp section a little too full and flat, the Philip`s Miniwatt`s helped to add a little richness and sparkle to the top end with out being bright.
Great sound from these Mullard CV4003 12AU7 NOS tubes in my PrimaLuna Prologue Premium preamplifier. They are a perfect upgrade from the stock PrimaLuna tubes.