I recently got a Portaflex head, and it came standard with the typical Ampeg high frequency hiss when I run it through the 2x10 Portaflex cab. I turned the tweeter off, so it's not that big of a deal. What bothers me though (and that I'm not sure is normal) is that when I plug my headphones into the head, I get the same LOUD staticky hissing sound when I'm not playing. It sounds like I have my ear pressed up against the tweeter with the gain on 10. Is this normal? I've never had an amp with a headphone jack before, and I was expecting it to be rather quiet considering it's a signal right from the bass.