Pyramid 903G Booster/EQ no amplification

I bought a Pyramid 903G Booster/EQ(60Watts RMS per side) and a 12V-6amp power adapter to make a cheap sound system to use with an MP3 Player for my backyard. I am using a pair of SONY SB1000 speakers rated at 8 Ohms wired to the front left and right speaker-outs of the 903G. I soldered a 5.5mm female jack/socket to the power wires of the 903G to accept the 5.5mm male plug from the 12V6A adapter. I wired a common ground 1/8 inch TRS plug to the two inputs of the 903G so I can plug in my MP3 player's headphone socket. The 1st time I tried it the sound would come through from the MP3 player's headphone jack when the 903G was off but would cut out completely when the 903G was powered on(the power light comes on on the 903G). I checked the connections and tried it again. Now the sound would come through also when the power is turned on on the 903G and let me shape the sound with the EQ but still no amplification - just the miniscule amount of amplification that the MP3 player's headphone jack puts out. I suspect it may not be getting enough juice from the adapter but the specs on the 903G states that it's fuse for the positive/red power wire is six amps so the 12V-6A adapter should suffice(?)