Hi. I would like to relay my experience, although I don't have any answers. I bought an EP-500, 2 M60ti's, 2 QS-8's and the VP150 last April. At the time I had an inexpensive SONY 100W AV receiver. I was happy with the system over all. However, I was quite disappointed with the EP-500. For the amount of money I paid for it, it just didn't seem worth the price.
Now, I had the very same experience of setting the EP-500 to 75 db and the receiver SW channel at 0db; the volume on the EP-500 had to be turned way down (6-7 o'clock)to acheive this. But the worst part was that at these settings there was very little bass produced. I played with all the settings, but when the bass levels got higher the sound became somewhat distorted, if that's the right word. I was expecting a gut wrenching, window shaking experience for the money. Well, I got the windows to shake but, like I said, it was not a very satisfing sound.
More recently I purchased the Denon AVR-4806. Well, let me tell you my whole world changed. Everything is fantastic! But the reason I am writting is to tell you bout the EP-500.
I ran the Audyssey setup and it set the subwoofer to 24 ft (it is actually about 8 ft away from the sweet spot but I left it at 24 ft) and it set the level to -8db with the EP-500 at 9:00 o'clock. Still, no earth shattering bass. But to make a long story a little shorter, when I manually set the subwoofer channel to +2 I finally got the sound I had expected. Unbeilevable! The windows still shake, but somehow it just all mixes in as part of the overall awsome effect (at least for movies). I also changed the crossover frequencies for the VP-150 and QS-8's to the upper frequency of the "Freq Resp +/-3db (Hz)" given in the Axiom specs and I set the crossover for the 60's at 80 hz. By-the-way, I have an Iso-max Sub 1-RR transformer in between the AVR and the EP-500 (for ground loops).
Now I have some ideas, but I am not sure how to explain all this. One thing seems certain though, when it comes to the subwoofer, a soundmeter is worthless; use you ears!
I do have a very strong warning for anyone considering purchasing the EP-500: when the EP-500 is used to its fullest potential, you risk alienating everyone else that lives in your house. My system is in the basement of my house. The EP-500 makes noises upstairs that I do not hear downstairs. And it can make some serious house-cracking noises. My wife and son are not very happy with that. Oh well, you can't please everyone :-)
