The NOS Mullard CV4003 (M8136) tube is a British military grade ECC82 / 12AU7 tube type. This 12AU7 tube has box plates which seem to improve resistance to microphonics and noise. They were produced in Mullard's Mitcham factory during the early 1980's.
The Mullard CV4003/M8136 tube has a deep and expansive soundstage, with overall fantastic definition. The lows are defined while the mids and highs are liquid smooth, just leaning to the warm side of neutral.