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!