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my speakers are rated down to 30Hz +/- 2db and greatest distinction is apparent on acoustic instruments, so I am not sure how important the sub is

Mine are rated to 20 Hz +/- 3dB and in my case, a sub made a significant improvement. Although my mains play low frequencies, they cannot play them as low or (and this is critical) as loud as my sub can. This is more important in sources with a lot of low frequency content, and less important when there's little low frequency content. Most action movies have a lot of it!

Good luck!

Will