These are famously used by Guns N Roses guitarist Slash in his Les Pauls, so you can have a rough idea of what they sound like from that...however, not all of us use Les Paul's, or Marshalls, or Vintage 30 speakers.
In basic terms, the Alnico 2 Pro bridge is classic rock personified. It's low output (around half that of a Duncan JB) but as a result lets the guitar breathe and brings out the primary tone much more than a high output pickup. Most aggressive pickups feature Alnico V magnets because of their deep, tight bass tones and present, biting highs, but the Alnico 2 Pro bridge uses the much softer A2 magnet. This means the bass is a little looser and spongier, with a strong mid and slightly rolled-off highs - great with Marshall type amps to stop them getting too ice picky, but also brilliant with more scooped Fender and Mesa amps that can use the decent helping of mids the A2P bridge humbucker gives.
Clean wise, this pickup's smoother and more middy characteristics give a bluesy, rootsy clean that responds nicely to volume controls. Split the coils with the onboard 4 conductor wiring though and you're in Vintage Fender single oil territory, which is extra versatile if this pickup is loaded in the bridge of a Strat or fat Telecaster. Add some crunch and you can get Stones sounds for days, or some lovely Who-esque, almost P90 flavoured grit. For more modern rock sounds, with some strong overdrive or gain the pickup copes brilliantly, sounding fat and thick while still letting every bite shine through. Kick in some extra middy overdrive and the sustain is great - lovely fluid solo tones are easily found here.
Possibly the only thing I wouldn't recommend this pickup for is hard edged metal rhythm riffing - it's too classic flavoured for that, and you'd be better off with active EMGs, Duncan Blackouts or indeed most high output Alnico V pickups. However, for absolutely everything else, as far as I'm concerned this is one of the best pickups Seymour Duncan have ever put out.
