How can a subwoofer expand the soundstage?

Roxy54 is on the right track. Notes at any particular frequency have both higher and lower harmonics. A system that isn't truly full range can't totally reproduce the lower harmonics. Adding a sub will allow reproduction of these harmonics which helps with the ambient info and spatial cues as mentioned by Roxy54. So, even music that doesn't seem to have much low frequency info can still be enhanced by the sub giving a better soundstage, imho. That's also the same reason why tweeters with excellent top end extension add to the spatial qualities of many recordings.

Rockyboy… I experienced the same phenomenon you describe when I switched my Theta Casablanca to run a sub in 2 channel Stereo mode with my Usher Be 718’s. Prior to that I was only running the sub for 5.1 home theater. Considering the fact that I already was crossing the loss pass signal to a phantom sub, I really wasn’t expecting image and sound stage to improve. The best way to describe the change is to say it was like setting the Usher’s free.

Stanwall, I am vey surprised to hear the subwoofer manufacturer recommends running the main speaker.
My understanding is there are tremendous benefits to be had by crossing over somewhere around or under 100Hz to a powered sub, even on quite good fairly 'full range' speaker.
Perhaps others can comment.

Psacanli,

What Stanwal said is correct (I believe) and what you say is also correct (I believe). REL does recommend running their subs in paralel to the full range speaker and the speaker be ran full range. This is done without any additional crossovers. REL subs usually don't perform too well above 50 or 60 Hz, but are outstanding in the lower frequencies.

On the other hand, to get the benefits you mention you need an external crossover before the amp to let it only reproduce higher frequencies and the speaker do the same. Doing this you prevent the speaker go into low freqs where many speakers increase distortion a lot. So you get more headroom on the amp + less bass-related distortion from the speaker, and then let the sub handle all the low freqs. However, to do this you need a different kind of sub that you can crossover at 80 to 100 Hz...like a JL (I hear).

I hope this helps. Please note much of the above is not first hand experience but rather learnings from reading this forum!

Regards,
Horacio

Well said Horacio. Being passive of course REL must advise running the main speakers full range. I'd withdraw my previous post if I could, it's not relevant here.