System gain structure. This happens when you have high gain preamp, high gain amp, and sensitive speakers in various combinations (usually 2 or 3). The signal-to-noise ratio of the preamp really matters here. Tube preamps and tube amps tend to have higher gains, combined with lower signal-noise ratios. Hence the hiss.
You can mitigate the hiss by adding inline attenuators between pre and amp (e.g. 10dB), and then raising volume to match your prior levels. This works "like magic" because you’re mitigating the noise floor from the preamp by re-apportioning some attenuation from before the circuit (the volume knob) to AFTER the circuit. In either case, you’re throwing away the same net amount of gain, but it matters WHERE this happens - because in the latter case you’re now also throwing away hiss noise generated by the circuit itself!
Then of course not all preamps are equal on signal-noise ratio - tube gear is especially diverse in this matter. I’ve had some super high gain tube preamps that were much quieter than lower gain tube preamps from a different manufacturer! The ARC Reference preamps have an exceptionally good signal-noise ratio for tube gear (I think it may be helped along by a few SS parts though). VAC preamps also tend to be pretty stellar here, and are pure tube.