SF Grand Pianos/REL Storm III combo - help

I have the REL stadium and Proac supertowers, so I don't know if this is any help to you, but: I have my subs at the lowest possible crossover point ('coarse' and 'fine' settings at the lowest level and the level at about 11 o'clock. In other words, try to find the point where you don't hear the sub at all and then bring it up a smidge. People say that one needs a month to find the right setting for a sub. It didn't take me that long, but be patient. You really want the sub to disappear ultimately, not draw attention to itself. By the way, my sub just sits in the corner or the room, a few feet behind and to the side of one speaker.

A lot of times, the RELs need to be set far below where you think the rolloff of the main speakers is "supposed" to be. In other words, if your mains go down to 40 Hz, set the sub somewhere in the upper 20's, and then play with it from there.

Having used just about every REL ST mkIII there is, and being a big fan of REL in general, I have to say that the Storm III is by far my least favorite. Compared to any of the others, even the Strata, it is exactly as you described, "muddy and abstract". This gets a little better with age, but not much. Hey, it's a bass-reflex box, comes with the territory. If there's any way you can afford it, the Stadium III is a giant leap upward, to say the least. If not, I would take a Strata anyday over a Storm. It won't go as low, but at least it sounds like music and not an indistinct thump.

Subwoofers, even the best ones, are good for not too good hifi !
It's defintitely a foreign body, especially in stereo hi-fi

There in lies the problem most all encounter, even when sellecting the best gear money can buy. Experience and knowledge are where it's at. And good acoustics, setup, tweaking will all be 2/3's of your sound ultimately, regardless.
You likely sit where you have bass challenges, and they correspond with same problems where you sub is, compounding room modes. Also, you likely have phase issues in relation to your speakers from where you sit, and probably volume matching problems...all compounded together.
I'd have to see your setup, and measure.
much more info is needed.
Consulting an acoustical/system setup expert will yield you 200% better results likely. Otherwise, you probably have KAOS with the rest of the speakers in the system, like most.
They all add up, for better or worse...often worse sonically, sadly.
Trial and error is your key if you do it yourself. It might take a lot of time...

how do you have the rel hooked up ? i really like the rel sub, but it sounds its best using the high level inputs..

the rca connection is less than optimal

i had really great results with a sonus fabers walls and a stata iii sub in my ht setup..( amps were muse mono blocks)...