oh, for the HK 3390, you have to take out the preamp/power amp jumper sa likod. This jumper connects the preamp and the poweramp inside the 3390 together
so in this case, the signal would have to go from the RA04SE (connect your ipod via rca in aux), then connect the preout of the RA04SE to the power amp input of the 3390 (i think they still call it main in - yan yung nakalagay sa HK3470 ko dati) then connect the LF to your 04SE (speaker a), HF to your 3390 (speaker a). make sure your volume control on both amps are on minimum and then turn on.
by the way, in this configuration, everything is controlled from the RA04SE including volume. The 3390 simply becomes a poweramp (the 3390's preamp functions are now disconnected so wala nang silbi yung volume control sa 3390).