You need to use matched pairs of tubes. Let the amp warm up and run idle for about 15 minutes or so. The amp needs to be at operating temperature with no signal. I always check bias after listening. Mute the preamp and wait a few minutes. The manual says to set the bias at 75ma per pair. ARC likes to run tubes hot. I would set the bias a little low, maybe 72ma. I ran my 6550 tubes at 62. It does not hurt to run the bias a little low as long as it doesn’t affect the sound.
I would call ARC with the serial number of the amp and see if it has ever been serviced. If the large caps are original, they need to be replaced. If the caps leak, they can damage the circuit board. It doesn’t not hurt to let ARC go through the amp and bring it to spec.