Andrew, using a "Y" at the receiver sub out is electrically equivalent to using a "daisy chain" through the sub. The sub arrangement is an internal Y with one branch going straight through to the next sub and the other branch going into the sub amp. That's why Randy got the same result with both configurations. In both cases the two sub amplifiers are connected in parallel with the receiver preout, dropping the input impedance to 1/2.
This would exacerbate what may have been an unusually low input impedance on the EP600 amp, as Mojo and I had discussed. The output impedance of the preamp and the input impedance of the sub amp are in series and form a voltage divider. Unless the sub amp input impedance is much higher than the output impedance of the preamp(as was said, at least 10 times is suggested)a substantial drop in voltage from the preamp occurs, with a loss of volume, when the low impedance sub amp is connected. The modified amp in the EP600 may have corrected this low input impedance(if it in fact existed)and now the connection creates no significant volume drop. The change to a 150Hz brickwall filter instead of 100Hz would allow for better blending with small speakers that need crossovers above 100Hz, but wouldn't in itself change the input impedance of the amp. Randy might check with Axiom as to the possibility of installing the modified amp.
