MM, Seems that Ref 6 may have a similar layout and tube complement as my former Ref 3... The 6550 tube rectifier in the power supply generates a significant amount of heat... The front paart of the preamp top plate is quite hot to the touch. Without proper ventilation, the device might go through tubes even faster than my estimate.
Conversely, the Rowland Corus that you asked to compare with Ref 6 is only moderately warm, even during summer time.
Regards, G.