I have the 550exr driving Maggie 1.6s. I bought the amp about a year ago and have been very content. Based on what other owners have stated in this and other forums that replacing the stock JJ tubes with expensive NOS tubes doesn't bring significant improvement, I haven't replaced the stock tubes.
I won't get into all the nifty audiophile terms to describe the "sound" of the amp, so I'll simply say that the Maggies sound wonderful, open and, as far as I can determine, completely unconstricted in their presentation. I have not been tempted to add a subwoofer; with the 550, the Maggies' bass is clean, never bloated or fuzzy, and if a recording actually contains well-recorded bass, the speakers will produce it accurately.
Before I purchased the 550, Frank told me the 350 would be plenty to drive the Maggie 1.6s, but I opted for the 550 because, as an American, I know that bigger is always better.
FYI: other equipment includes a modded Jolida 100 CDP and the VTL 2.5 preamp with NOS tubes.