Hi JerryL

Thanks for your input.

I'm sure that the sub is corrected perfectly - if it wasn't surely there would be a lack of bass on all sound presets (not just Trifield)?

The speaker level splitting of hi/lo frequencies isn't an option as the RELs have no high pass filter.

I tried the sub with speaker level and line level and it didn't work (as you said, overlapping bass, bass cancellation etc).

As I said in my original post, I set the levels using an SPL meter and using the 561s built in test tones. I have also rune the sine wave and the response seems fairly even (my room has no acoustic treatments, so I would expect the odd dip/peak).