I hope my brain doesn't so obsessing over this...
Offhand, the first thing I think of was the early song of what can't they do when they're together...and the rest of the season is them falling apart (the fractures showing even in this ep). So he'd feel that HARD. It could even include Tara's death, though references would be cryptic enough as there would be "too much interference to work through."
Technically, if returning Buffy to life allowed the First Evil to start a power play, he should sense that when Buffy sings, which might be the most dangerous thing for him, directly and indirectly.
His hearing Spike sing...and getting visions of him and Spike working in Wolfram & Hart (at least Lorne is)...might cause his own near-mortal song of horror (being what he is, he can probably survive Sweet's power...though his singing in such a case would probably kill anyone near him).
Most of all, I think we could get a definitive statement about WHO actually summoned Sweet. Because the official line is...well to be polite, let's just say I don't buy it as anything other than bad writing meant to make a gay joke which they deemed more important than the story itself.
But if he said it, especially with a reason other than was given in the ep (he'd certainly see Xander's past experiences and resultant caution of magic before this), then I could buy it was Xander. But better still if it's more creative. (Heck, maybe even him saying, "I'm not sure...do the words 'Tucker's Brother' mean anything to you?")
While we're at it, maybe a statement on what Dawn is exactly. And maybe a bit of the duo-realities (more detailed than when Buffy did that ritual that momentarily let her see through the monk's lies). That could be interesting.