I would highly recommend against paying $400 for them (
Senn PC360 is $166 right now at Amazon... ).... And a USB sound card is exactly that, a USB sound card, not something I tend to recommend unless you are in absolute need and are unable too occupy PCI-E/PCI slots.
You can purchase the Beyer sound card seperate,
http://north-america.beyerdynamic.com/shop/usb-sound-card-for-headsets.html, but I would really recommend a SoundBlaster Z at this time due to it's price/performance. You can of course use your onboard, keeping in mind the MMX300 does NOT need an amp to be driven.
"Best" around is controversial, you have the ASUS Xonar Phoebus, ASUS Xonar ST, Creative TitaniumHD, Creative ZxR, and HT Omega Claro XT. As you can see from my specs I run the Phoebus, nothing but good to say about it and since Creative put out some decent drivers for their Z-series of cards recently I'd say the ZxR is an excellent pick that can be less finnicky than the Phoebus. Of course there is still the TitaniumHD that is to this day a fan-favorite, although many of them have been dying after prolonged use.
There WILL be someone to say why not a receiver and digital, quite simply why ruin the sound from the source using digital? Analog can be looked at similar to vinyl in regards to sound (if you don't understand, go listen to some vinyl), so my personal recommendation is to stick with it. On top of the fact that you will be spending quite more on a quality receiver that takes up space at your pc for no reason when the sound card offers the superior DAC anyway (assuming you don't purchase a high end receiver of course) :/
As far as amp/dac, Schiit offers some great product! I personally moved from a Schiit Asgard and Bifrost to a Matrix M-Stage amp/dac stack to my current WooAudio WA7 Fireflies. All have been excellent, but my favorite so far as been the WA7 paired with the Beyer T90. The WA7 and T90 happen to pair together with excellence. If you want something great for the price, look at the Schiit Vali / Modi stack, amazing for their price tag.