Are you actually using the zx300 w/ the Z7? You think the zx300's max balanced output of 200mW / channel is enough to do the Z7 justice in this regard? What's your average volume level (out of 120) on the zx300 with the Z7 on the balanced out, high gain? Reading in this thread and also the reviews in the Head Gear section here, a lot of people claim the Z7 needs oodles (like, tabletop amp levels) of power to sound best. (The obvious response is that all good headphones do, really) The pha-3 that Sony pushed alongside the Z7 when they both launched outputs 320mW / channel. The WM1 series does 250mW / channel. But, on the other hand I read a professional online review somewhere that said the Z7 was nicely driven by the WM1A on the SE jack, and that's only 60mW / channel. I've been planning on buying the zx300 but this foggy info has actually had me considering the WM1A recently, despite the considerable price and size/footprint difference. Not just for the Z7, but other headphones in the future.
On the other hand, if I could go with the zx300, with confidence it's got the oomph and then some for these headphones, I'd only save about $450 or so.