Geno, welcome. I don't have personal experience with the EP500, but you should study the recent posts here by delighted 500 and 600 owners. As to the DefTechs, it's unclear under what conditions they measured and how far down in level it was at 13Hz, but that number should be taken with more than a grain of salt. Noted subwoofer authority Tom Nousaine tested the Supercube I as part of a system and reported
these results and comments. Note that the maximum extension was 25Hz at only 78dB and wouldn't appear to meet the requirement for "plumbing the depths".
