Subwoofer doubts: crossover setting for 60hz speakers


Hello there. I’m about to add a subwoofer through “high level” inputs to my stereo system, and I have some doubts. Nominally my speakers (Cabasse Riva) go down to 60hz. According to the stricter books, this means I should set the sub’s crossover to 120hz. Which would be a problem, since -from what I read- sound becomes “localizable” over 80hz, and my only spot to place the sub is a corner of the room. I guess that would severely affect soundstage, which is a deal-breaker for me. 

So my questions: 

1) Would it be ok if I set the sub’s crossover to 80hz? or would I be risking to create some kind of “gap” between sub and speakers? Do rememeber that I’ll hook the sub through “high level” inputs, so there'll be an increasing overlapping of frequencies above 60hz. 

2) Or would I be better off adding 2 subs instead of just one and setting them at, say, 100hz? Many audiophiles on the web advocate for double subs... 

Keep in mind here that my current options are: 1 excellent Velodyne spl1200r (that reaches 20hz), or 2 fairly good Focal sw700w (that reach only 40hz). If the answer to question 1 is "yes" -ie. if I may set the sub's crossover to 80hz without noticeable gap and preserving soundstage- I'll definitely go with the Velodyne.  

Thanks for ANY help -I need to move fast!


fedocable
Thanks a lot, Tim. Extremely informative, just the kind of feedback I was hoping to get. I'll go with the Velodyne, no more doubts.
IMHO, the main reason so many people have issues integrating subs into their system is that they try to cross them over WAY too high. If your -3dB point of your speakers is 60 Hz, I'd begin experimenting with a sub crossover point of 0.7 x 60 Hz = 42 Hz. Unless you are using a Vandersteen sub, you want the sub to just come up underneath your monitors...and usually not all the way up to the -3dB point of your speakers. 
IMHO, the main reason so many people have issues integrating subs into their system is that they try to cross them over WAY too high. If your -3dB point of your speakers is 60 Hz, I'd begin experimenting with a sub crossover point of 0.7 x 60 Hz = 42 Hz. Unless you are using a Vandersteen sub, you want the sub to just come up underneath your monitors...and usually not all the way up to the -3dB point of your speakers.
nrenter has it right if you are looking to add to what the main speakers can't physically reproduce. That's a good starting point and may need to increased. By setting the sub to 80hz you will add the lows your speakers can't do but also add to what they will do (huge hump in the 50 - 90 hz region). But if the sub is out of phase with the mains you'll end up with a suckout in that 50 - 90hz range. Phasing is very important because you don't want the added lows leading or lagging the lows from the main speakers. I have a Rel B3 which has a phasing switch - 0 or 180 degrees. So you may have to move the sub to get it correct. A couple of inches can make a pretty big difference.

That said my speakers are rated to 40hz and I set my sub to 32hz to get the best blend when I use the sub. I say when 'I use the sub' is because when I stream from the internet music the lows are already exaggerated.
Thanks a lot, xti16. I'm already free from the annoying idea of setting the crossover as high as 120hz. I'm convinced 80hz or even less will work fine. That means I'm going with the single subwoofer option.
As for it, I just found a third challenger, at a very nice price: a Phase Technology "Power FL-10"; comes with a 10' woofer plus a passive radiator. It goes down to 26hz, decent enough. The seller claims it's new. There are a couple reviews on the web, and they are all very flattering. And, best of all, the price is cheaper than retail in USA! -that is quite extraordinary here in Argentina.
So, new question: has anyone heard of it, or at least of Phase Technology? Thanks in advance!