Any sub must have EQ, variable filter orders, continuous phase, invert and separate music and LFE input if using with HT.
If the system does not have active DSP, then getting it in the sub does wonders for integration.
The Martin-Logan X series tick all the boxes in a reasonable price range.
You should roll the bottom out of the mains to improve dynamic range. This is simple with a first order passive filter.
see ieLogical SubterraneanHomesickBlues for a primer on integrating subs
Twenty years ago REL were top drawer. Today, a geegaw marketing outfit whose verbiage is mostly malarky. FanBoys love the bling.