Magnatone used 6GW8 tubes in the M2, M4 and for the reverb output on the M15. This was a neat audio tube that combined a power pentode and high mu triode in a single glass tube. This allowed for two push-pull power pentodes and the extra two triode sections could be used for the phase inverter, thus reducing tube count from three to two for the power section on the amp.
The 6GW8 tube is long out of production, and was used in many HIFI amps of the 1950s and 1960s. The european designation is ECL86. 6GW8's can be very pricey.