It's likely true that a dedicated headphone amp is going to sound better than the amp built into an integrated amp, at least if you spend a reasonable amount of money. I had a very nice Headroom amp with the Sennheiser 600's for awhile and it sounded great but I found myself not using the headphones often enough to justify the amp.
To your question about considering the impedance when picking the phones, I would think what's most important is that you find headphones you really like first, then if you find you enjoy the experience, find an amp that matches the phones. Because of the issues of fit and the clarity of the sonics, opinions about what makes a great pair of phones seem to be even more personal than for other gear.
To your question about considering the impedance when picking the phones, I would think what's most important is that you find headphones you really like first, then if you find you enjoy the experience, find an amp that matches the phones. Because of the issues of fit and the clarity of the sonics, opinions about what makes a great pair of phones seem to be even more personal than for other gear.