Voltage divider to drop volume of one speaker by 1.5dB
As Tony mentioned, the configuration you're looking for is called an L-pad attenuator (or for its balanced cousins, the H-pad, U-pad, or O-pad).  These are pretty easily built in-line in XLR barrels.  

1.5dB is small enough that all the online calculators I could find were giving negative resistor values, so you'll probably have to do the calculations by hand.  This page explains the calculations you'll need to select resistor values: http://www.uneeda-audio.com/pads/.  It also has info about building the circuit inside an XLR housing.