Using the MMX 300's right now, and they are truly excellent.
Sound just like the Beyerdynamic 770's...which I always liked. I'm coming straight out of my work PC, a Dell with the stock sound card. The MMX 300's, plugged straight in, are nothing to write home about. Thin bass, ok sound. But with the included USB sound card? It's like having a little 3Move at my desk! Powerful, deep bass, great mids and highs...and well, everything the 770's should be.
BTW...Beyerdynamic should sell that little USB dongle sound card. It's amazing...like a pocket DAC. I want a couple more! Seriously, I'd like to try it out with my other HP's, and see what it does for them, too.
As for my Skype issue, I discovered that Skype automatically mutes the mic when you hit 'call.' If you open up the control panel's audio settings, and uncheck the microphone's 'mute' box, you can hear your voice perfectly on a call.
I'm looking for a way to make Skype not automatically mute the mic..but until I do, unchecking the 'mute' box is an acceptable workaround.