There may be another issue here. Possibly a simpler one? Is the sub connected to your receivers LFE/Sub out via an RCA connection, to the Sub's Line IN?
Verify the settings in the receiver show the Sub is present/connected so that it will send a signal out to it.
Is the light on the back of the EP500 green?
Can you run the yamaha receiver's test tone that goes from speaker to speaker, and verify you do hear the sub working.
Just checking some basics, as 6-7 o'clock is plenty loud for most of us with the EP500.
If you are sure you are getting signal out of your sub, but it just seems a little "low" then turn it up to your liking. Everyone is different. Sorry I don't have experience with the Yammy 2600 to give you more detail.
