Newbie - help required Please

Well, this is a little bit of a hard choice sense all of those aren't very good (in my audiophile opinion). If I had to, I would pick the Panasonic ones. The HD201s have volume troubles (not being loud enough) and the K450s I just don't like because I've always hated on-ear headphones. Plus, the bass on them is extremely overpowered and it overshadows all of the other frequencies. The Panasonics are a little bit more balanced and have the best sound of the three, but they don't isolate very well so if you need them for traveling purposes, they won't do very well. But that would be my choice.

I've never really tried those, so I don't know if they'd be better. If I were you, I would actually consider thinking about the Sennheiser HD429. You can find them for around $80 retail, but they're listed here for $60. You might want to check them out, and I think that they're better than all 4 of those that you have mentioned. They have the most balanced sound out of them, and I think are probably best for what you like. They don't have a lot of bass, like a lot of Sennheiser supra-aural headphones, but equalizing it can help a lot. Again, these don't provide any isolation either, due to their open-back design.

For the $30-50 price range, I'd get the Panasonic RP-HTF600. They're good headphones for the price and can be driven by any portable devices. Keep in mind that they're semi-open though the sound leakage is not very noticeable.