There are some things gained and lost in a chamber version of A Little Night Music. The grandeur and romantic sweep of the original production, with its Boris Aronson sets, Florence Klotz costumes and full Tunick orchestration, is missed, as well as a certain lightness in touch. But I found a lot to like in Trevor Nunn's production too and was not disappointed in it. Lansbury, Zeta-Jones and Hanson are all very good, as is most of the supporting cast. The score remains a marvel of wit and style and it's a good-looking production in its own right and the set is rather ingenious. Jason Carr also works some miracles from an eight piece orchestration and the sound, while admittedly smaller than the original, feels appropriate to the scale of the production in a 900-seat playhouse.
In the case of Sondheim revivals, some productions are in an unenviable position. The productions that were startlingly different, like the Doyle-directed Sweeney and Company (like them or loathe them) or Roundabout's Assassins, which benefited from so few people having been able to see the original, got the reviews because there wasn't as much as basis to compare them to the original incarnations. The 2008 Sunday in the Park got much better reviews than the original production, although I think that was a case of the critics finally catching up with a show they didn't get the first time around. But the revival productions of Follies, Pacific Overtures, Into the Woods and now A Little Night Music have all been found wanting. Now these shows all had various assets and debits, but I think in particular, they are/were also competing with people's memories (real or imagined) of the original productions.
I think in approaching Sondheim revivals, it's best to love and admire the originals, but try to meet and appreciate the new productions on their own terms.
Begin at the beginning and go on till you come to the end: then stop.