As with most things vinous it comes down to personal taste and preference. I probably drink Champagne seldom enough to properly notice the difference dosage causes between bottles. I also believe the bubbles prevents me from analysing a Champagne as forensically as I would a still wine. The other thing would be what went into the blend and, with the Souverain it is 50% Chardonnay and 45% PN and 5% PM. I find PN adds a richness to a champagne, especially when compared to a BdeB. Could that be mistaken as extra dosage or is it much easier to tell where the richness/sweetness comes from?
However, I see the Souverain has a dosage of 'less than' 8g/l, which is the same as the multi-variety Champagnes I'm used to drinking from Gaston Chiquet, so I'd be surprised if I found the Henriot a tad sweet. However, on checking, even Gaston Chiquet's BdeB has 6g/l and I find that very dry (but not as dry as some of Francis Boulard's zero dosage Champagnes).