Fatparrot, the Audio Research website does not use NOS tubes in their products at the factory.
Below is the exact language in the manual regarding tube life expectancy and the use of the muting switch:
"For superior sonic performance, a warm-up period of one hour is recommended. In addition, our PH3 may be safely left "ON" contiuously for maximum performance at all times, but at the expense of higher maintenance costs (more frequent tube replacement). When not in use, simply use the "MUTE" switch. The PH3 draws less wattage continuously than a 60-watt light bulb."
"The vacuum tubes inside your PH3 are 6922 dual triodes, and with normal use should not need to be changed for approximately 2,000 hours of use."