To tell if the tubes are bad....turn it on, hear any noise? How far do you have to rotate the volume control before you have noise? Connect (or turn on a source) how does it sound. Dull, rolled off. Perhaps your tubes are over the hill and could use replacement. Bright - glary? Perhaps you might like a warmer tube. You should have working tubes in all sockets including the phono sockets, however the phono socket tubes can be in any condition so long as they are drawing current. If they are working the only tubes you would need to replace are the line stage tubes. As to which brand of tube, read Job's tube lore in AudioAsylum FAQ's for a description of the tonal quality of the various brands available as NOS and current production tubes. Consider the sound you would like to hear and then post here and in Audio Asylum and ask what other ARC owners have used to produce the sound you are looking for. Hope this helps a bit. Oh, also do a search in the tube forum in AudioAsylum, this pre-amp is discussed quite often.
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