I just successfully flash my Aztech DSL600EW router's firmware with Router Tech Firmware.
First upgrade firmware to official firmware 66.53.2-002 , if you need to download one , you might be able to find it at modem help web site.
Then download the Router Tech firmware - routertech-ar7wrd-adam2-firmware-4ports-20100105.zip , I tested with this version and it is working.
Using the web interface to upgrade the gateway reboot and run.
The LED configuration might not correct by default , so telnet into the router , issue command : setenv led_conf led.sar600 && /sbin/reboot , the led colour should be back to normal.
In case if you flash the firmware wrongly, you can refer to this URL : http://www.techarp.com/showarticle.aspx?artno=374&pgno=2 to restore the router back to official firmware.