It depends on where that signals intelligence is gathered. If abroad, then fine. If during the course of signals intelligence gathered abroad a domestic phone number, etc., arises, then a warrant can be secured to pursue that lead. By way of contrast, domestic signals intelligence would be flatly unconstitutional sans a warrant - a warrant is required for domestic law enforcement purposes; the president has no domestic warmaking powers in the absence of civil war, revolution, insurrection, or the like.
In other words, if conducted domestically, signals intelligence becomes, on the face of it, a law enforcement issue that must pass constitutional muster. If the courts have decided otherwise, then they have decided poorly. Nothing remarkable about that as the Kelo and CFR decisions among others in recent years have amply demonstrated.
It goes without saying that I do not concur with your assessment that the post 9-11 congressional resolution is a declaration of war. Rather, it is a prescription for permanent government employment of emergency powers because war against an unidentified and undefined enemy, against a tactic rather than countries and people known to promote, foster, and practice it, is inherently unwinnable.
Let’s go back to my World War II analogy for a second.
If we intercept a phone call from the German High Command to a Nazi spy in this country, is it Signals Intelligence, or is a law enforcement matter?