The 5AR4 are indirectly heated and provide a smooth (slower ramp of current) start up of the amp. I have also been told that they present a lower impedance to the power supply and therefore provide better dynamics. I use them and felt they improved the transparency of the amp. I was also told that there is no need to spend big dollars here on NOS tubes, but to each his own, instead go with a pair of the lesser cost current production manufacturers. Certainly worth a try IMO.
Chuck
