NR509 and EP1500, not playing nice, preamp sub output too weak?

I bought an NR509 awhile ago with the intention of eventually hooking a subwoofer to it with a EP1500 I had laying around. I got around to doing a quiet fan mod to the EP1500 so it is acceptable to use in the living room and hooked it up. I ran into some problems.

I turned the subwoofer on in the receiver settings, hooked an RCA to the subwoofer out on the receiver, attached a 1/4" adapter to the RCA, and plugged that into Input 1 on the EP1500. I set the gain low, booted both systems, and put in some music.

The subwoofer did not work. I adjusted the gain up, but it just produced a buzzing noise so I lowered it and went back into the settings. I adjusted the front speakers down (-12db) and the subwoofer up (+12db) to try to up the preamp output relative to the speakers.

I fired it back up and still got buzzing with a faint amount of music through the subwoofer. Turning up the gain increased the noise as well as the music level.

To eliminate the amp and sub, I hooked up my iPhone directly to the input on the EP1500 and it played music perfectly fine without any noise.

I then bridged the amp to pump up the output and it didn't help any.

I then decided to run Audyssy and it ended up determining the same settings that I had (sub fully boosted and speakers cut.)

So, I've come to the conclusion that the preamp from the NR509 is simply not strong enough to drive the EP1500. I've come to you guys to help me determine what my options are. Preamp processors seem to be pretty expensive and I'm not familiar with line boosters.

What would you guys do to fix this problem and do you agree with my diagnosis? I'm open to any ideas.

Thanks

To connect the iPhone (which works), I used a dual ring TRS into a mini-TRS that connects to the phone. For the receiver (that doesn't work) I use a mono TRS into RCA into the receiver.

I do get some sound from the receiver, but the hummmmmm is louder than the sound. The iPhone does not hum when connected.

I messed with it more last night and believe the adapter (TS adapter to RCA) is not working properly. I moved it around in the jack on the amp and it would go in and out of working and not working.

I ordered an XLR male to RCA male as the amp has inputs for XLR. I will give that a try and report back.

Thanks for the suggestions.