How can a subwoofer expand the soundstage?

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.

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153

Horacio,

Your observatons comport with my experience. I use KEF R104/2s with a Velodyne HGS-15. Crossover by the pre/pro is set at 80 Hz. The 104/2s sound fine full range by themselves, but I think they have a bit more transparency when relieved of LF duty by high passing. OTOH, when both the 104/2s and the sub cover the same mid-bass region transparency seems to suffer just a bit.

db

"I thought its because the subwoofer is omnidirectional in the lower region and thus adds to and expands the soundstage that the conventional speakers, whose sonic signature is directional, can't possibly convey."

Omni=directional speakers apply this same concept and resulting sonic effects to higher frequencies as well.