The P9's are rather expensive after looking them up. You might consider the
Sennheiser 600. I have the 650's and they're very nice. Some people and reviews put the 600's above their newer brother the 650 as sounding better overall. For the price you can't go wrong. I also have the
Sony MDRV6 headphones which are very reasonably priced and sound very good as well - when hooked up to a quality amp and DAC. If you go for the Sennheiser's or any high ohm set of cans you will need an amp and DAC to power it properly. In fact, after going down this road and asking the same questions myself very recently, I highly recommend an amp and DAC - highly recommend.
If you're in the United States, I was pointed in the direction of Schiit Audio for headphone amps and DAC. The models I bought from them are amazing and I am very happy with them.
I went with the
Magni 3 amp for $99 and the
Modi Multibit DAC for $250. You could also go for the Modi 2 for $99 if price is a factor for you. Don't forget you'll need to buy a few cables to hook everything up, but the cost is minimal.
At first I just bought the amp and used that and used the Mac mini as the DAC and the performance was still wanting. Then I bought the DAC and added that to the chain and it made a HUGE difference in power, quality and overall sound.
If you want to read some reviews on the Schiit and other amps and DAC's check out
The Master Switch website. I also found
Headphone.com to be a good source of information, comparisons and reviews and you can leverage them, and other sites, to help you make an informed decision.