Hi Larry,
I have the EP-500 and some folks have/had both the EP-500 and the 600. If you care to look at the objective differences between the subs, then
this table provides some nice comparative information. Basically, the EP-500 has a 500W amp, handles 17-100Hz, and has a max anechoic SPL of 109dB. The EP-600 has a 600W amp, handles 15-100Hz and has a max anechoic SPL of 111dB (difference of 2dB being 1.26x1.26=1.6 or 60% more acoustical energy). You may want to audition these subs to get an idea as to their differences, but I think people who own either or both have been uniformly pleased.
John