I recently purchased a digital piano (PX-330) that has a built-in USB interface as well as a MIDI IN/OUT. I also have a Saffire Pro 24 DSP audio interface. Should I use the piano's USB interface or should I use my Saffire to connect it to my computer?
I doubt it makes a difference, but for what it is worth, I would more likely trust an actual midi cable, and go from the midi out on your keyboard to the midi in on your interface.

I have a midi to USB cable, and it might be because it is a cheap one, but it doesn't work a quarter as well as my old midi out to midi in on my interface. (I replaced my interface to one that doesn't have midi, so I thought using midi to usb would be a good solution...)

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.