Best Headphones & Amp---Opinions Please

I have a B&W / Lexicon centered high-end system. Just discovered the joy of listening to headphones. Have the Senn 650's with an upgraded 15 foot Cardas cable (no headphone amp) connected directly to my Onkyo 905 receiver. I would like to "push the envelope" in this area by upgrading to whatever my fellow aduiogoner's may suggest.

I would like opinion on the best equipment available. Cost is not a major consideration, but, I would rather not spend considerably more just for a 5 to 10% improvement--in other words---the most bang for the bucks.

Thanks in advance for your assistance--Dave