Also check
www.dact.com they make some pretty good ones too.
If you are really in to soldering one up from scratch you could look into the stepped attenuators from elna make sure to buy the make before break version. and of course top notch resistors. the ladder type is the best for audio quality. but i have not met anyone who could tell the difference in sound. the fact is the audio passes through only one or 2 resisitors in a ladder type. In a series type it will pass through more resistors if you attenuate more.
I saw the site and what is that tube doing there on the picture? and there are lots of condensers and ic's on the boards check if they not ruin your signal path if they are in you might be better of with a good old rotary switch.
wich does not necceserally mean that is is bad if they are used just for switching it is no prob.
also check if the setup is right for passive volume control and take out the pre amp you are using and check if you can get the signal loud enough (CAREFULL IF THE POWER AMP IS GOOD THIS CAN GET LOUOUD)
greets!