I was thinking 'Calibre' by Kris Sowersby could be a nice compliment?

I think Calibre is a nice alternative to Futura, but not as a complement to it.

Since it's the logo in Futura, there's no way to know until you try it.

I once had success talking a client out of Futura for web copy (Futura at 12 - 14 point on screen is horrible) and we went with Avenir. Avenir at that size, on screen, felt Futura like, but didn't have the problems Futura gets at that size.

The company I work for uses Futura as one of its identity faces. We use Akzidenz Grotesk as a complementary sans serif. Not "modern" per se, but we do use the pairing regularly.

Typeconnection.com suggests ITC Century.

[Oops. Just realized you were asking for a san serif.]

Univers?

It would depend on several factors. To cite a few:
Logo is all caps, mixed case, lowercase?
Weight(s) of logo?
What the client do?
Which is its style?
For what applications would be needed the complementary face?

Futura is such a classy sans-serif face, another sans-serif face would diminish it, not complement it, unless the two faces are radically different in some other way. Try using a serif face to complement, such as Garamond or Bodoni. Bodoni text with Futura Light headings is a great combination. http://www.smashingmagazine.com/2010/11/04/best-practices-of-combining-t...