I have a Bryston BP-6 preamp which I've owned for years and is well broken in. But I've just recently began using the headphone jack when I bought a pair of used Sennheiser HD600's. I'm at least the 3rd owner of these cans so I'm assuming they're well broken in. I wasn't all that impressed when I first plugged them in but I find they've really improved as I've put some hours on them. So is it possible that there would be a break in period for a headphone jack that has never been used. Just curious.