It depends a little on what you mean by dB.

You probably mean dBu or dBv (I say this because dB is a relative measure, rather than an absolute one, so is not helpful here). See wiki.com for more information on dB, dBm, dBu, dBv, dBV etc).

In this case , the full level output of the H8000 is about +26dBu, so if you are measuring -80dBu with no signal, this suggests a signal to noise level of about 80+26 or 106dB, which is reasonable, depending on the measuring conditions.

Hope this is not too technical. What it suggests is that whatever you have after the H8000 is too sensitive, which is why you are hearing the noise. You are probably also not driving the H8000 with enough signal (the top green led needs to light from time to time.

Once you are driving the H8000 hard enough (use LEVELS/inputs/Pre A/D Gain if necessary) then adjust the LEVELS/outputs/Post D/A Gain to avoid clipping whatever follows the H8000). When all this is done, you will have a dynamic range well over 100dB, and are unlikely to hear any noise.

Essentially, you could have the world's quietest unit (the H8000 is quite good), and if you feed it into a sensitive enough input, you will hear noise. If you then turn up the gain, it will sound worse.