With the y adapter you were using you were sending 2 unbalanced signals Left & right to the balanced input of each speaker. When you do that all you will hear is the difference signal as in L minus R not the real signal that is supposed to drive the amp. That will cause the signal to sound very weak. If you feed a true balanced signal to the balanced input you get double the voltage swing which is actually 6 db gain not 3db providing you are feeding the + and - of a differential input amp however this can be compensated for in the circuit resulting in no gain if desired, just an improved interference & noise spec is all. The BX8a probably uses a differential amp at it's input. My earlier BX5's do.
Sounds like you got that fixed now though. Just thought I could give a better explanation as to what was really happening to cause the weak sound.
The proper way to feed an unbalanced signal to the balanced input is to use a RCA to tip sleeve I.E. two conductor phono plug, not tip ring sleeve which is 3 conductor phono plug. The long sleeve of the tip sleeve 2 conductor plug grounds the unused input resulting in lower noise.
Edited by germanium - 3/30/12 at 3:45am