Trafo's=coloration. A passive type like this is the least coloring if the previous and the folowing stage have no problems with the impedance.
I build a design from the Elektor audio special (dutch magazine) Wich has also relays for the switching of the resistors 64 1dB steps remote control and a very clean 15dB amplifier.
http://www.just-sounds.nl/fototjes/relaixed.JPG
Works great but no sub out like yours.