Tex,
Take Five, besides being one of the jazz masterpieces, is exceptionally well recorded with great range and detail. There is no mistaking the deep bass of the kick drum and snappy snare in the two minute drum/string bass bridge.
I'm going to rule out a problem with the EP600 since you appear to have replaced most parts. Though it is still possible the enclosure itself can be causing undue resonance. In this regard, try lying it on its side and putting heavy articles on it on the middle of one side.
My inferences as to why you are preferring the Sony:
a. Reverberation time as a function of frequency in your room. Your room is untreated with high resonance. Depending on how it is interacting with the subs phase at different frequencies, you are going to get cancellations and reinforcemnts. Your particular untreated room is meshing better with the time/FR characteristics of the Sony sub/M80s at your listening position.
b. You are less sensitive to the higher distortion from the Sony. Distortion in subwoofers is something more easily tolerated than distortion in the midrange or HF.
c. Improper placement of the EP600. Every room is different but in most rooms the sub/sat system is phase aligned when the subs are next to the speakers. The more out of phase the sub is with the mains, the less tight and mushy bass sounds with more frequency irregularities. As an aside, that's the main reason I advise people not to use Audessey or like auto equalization. FR is improved at the expense of time misalignment.
d. Improper integration of the EP600 around the crossover point as a result of the above mentioned phase and room acoustical interaction.
