Schu... pm me your email. If you are able to do some soldering, I have an application I got some years ago. It runs in Windows, doesn't install but just runs. You put in your input impedance and output impedance and the amount of attenuation you want (1 db increments). It gives you the schematic and resistor values to wire it up. It sows for balanced and unbalanced attenuators. example in the pic...
I removed both - speaker and passive.
The amp is in the middle of the box, between the "speakers" - at least it was on the RSW-15. Maybe you could get way with only removing one or the other- i dont recall (age :-) )
On mine the rear one is the speaker, the front is the passive.
They are held in (besides the screws) by a thin tape, easy enough to work free with a little bit of patience, prying etc.Same story for the control plate, remove the screws, and pry up gently.