check for feedback resistor at pin 18 and 21 going to pin 6, if their is a resistor their, try to increase its value to 2x to 10x, if its not working or their is none their, then the gain must be fixed and embeded.you can also try making ur own feedback its worth a try.
ant_B at pin 18 isnt even connected to anything .. ant_A at pin 21 has a resistor inline with it and thats about it, not sure its working the feedback from there, the gain shoudnt be fixed because thats what the power dectector is for to throtle the power ... although changing the power to 25% from 100% in setting seams to make no difrence .. so might be fixed .. or there is a voltage comparitor its using somewhere to fix the voltage .. but without having all the data sheets im never gonna know ..... think its a dead end now .. well on this usb anyhow
but i did find this
http://www.solwise.co.uk/wireless-indoor-wl-ipa2.4-booster.htmthey also sell amps up to 4watt but i wount go that far
also found this about homemade amps
http://www.zero13wireless.net/foro/showthread.php?t=254