As others comment, allowing for for easy bias adjustment with a built in meter is a good feature. But note that (1) some tube amps use LEDs that go dark when the amp is correctly adjusted (easy to do) rather than a meter (e.g., the Conrad-Johnson MV-60) and (2) some tube amps are automatically self-biasing with internal sensing circuitry so the user doesn't have to do any bias adjustments at all (e.g., the BAT VK-55).
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