First, I don't know that amp well enough to say if the noise is "normal" or a common problem with a known fix. But I've heard the noise you describe from failing opamps and transistors on older gear. Since the noise worsens with volume you can narrow it to the preamp. A quick shot of freeze spray often temporarily quiets a noisy chip or transistor. A short blast on each preamp silicootie may reveal a noisy one that could be replaced.