If treble clarity is what someone is looking for, I think Z7 will be a disappointment.
Having owned the TH600 for 6 months and auditioned the TH900 for many times, my main gripes with them is the mids suck-out, which I can't get used to.
Value-wise, I couldn't agree more with you as the TH900 can be had for US$1.1k-1.2k nowadays, while it's younger brother the TH600 can be had for US$7xx.
Bear in mind that when the TH900 was launched, the RRP was US$2k, before they revised it to US$1.5k much later.
The Z7, for me should be priced within the US$500 price-bracket, as that seems to be the going rate of Z7 in Japan.
I bought the Z7 for less than US$600, so I think it still represents a good buy as within this price bracket, not many headphones can make me smile as the Z7, considering I already own the HD800 and LCD-X