I changed the bulbs in my DE headlights and it improved them no end, xenon bulbs do degrade over time. It's headlights coming towards me I have a problem with
Bi xenon are fitted to the later HR model 350z and very hard to come by second hand so good luck finding them. The xenon lights are a lot easier to get hold of at a reasonable price.
Well after wanting these for a year or so I've finally bought a set (thanks to Chris at Tarmac sportz). They arrived Friday so after a quick rattle can paint job I fitted them today. They still need cutting back and polishing but I'm impatient
Do not give them anymore money. If it's not sorted when you get it back from them this time I would take Alex up on his offer to diagnose your problem , he knows his stuff.