I've read somewhere before that there's a high chance the dual rectifier sounds we so come to love from metal guitarists who used them are either modded, or were tracked with a different amp in studio albums. I've never personally used a real dual rec, but from the stroies i've read so far a couple of people were puzzled when they 1st tried a real dual rec because it doesn't sound anything like what they heard in those metal albums.
the closest i've come in contact with dual recs are their simulations on mfx like the pod x3 and pod hd. they sounded the most distinct amongst the high gain amps for metal in the sense that they are the thinnest and harshest amongst the lot, but also very tight and not that boomy. i've also heard some axe-fx and real fairly good mike up samples of the dual rec, and i must say they sound quite different from say, john petrucci's tone in the albums he used a dual rec, or even in this video:
http://www.youtube.com/watch?v=2YWXpHl9sGk (can somebody verify if it's a dual rec or a mark iic+ there? looks like a dual rec in the bg. it sounds very different from the album version which was a mark iic+)
here are some examples of both their modelled versions and real sm-57 mic'ed up versions. i assume these aren't based on modded amps.
http://www.youtube.com/watch?v=YPlPzuiMZOU&hd=1
http://www.youtube.com/watch?v=76S3E-wEgYY&hd=1
quite a nice tone for riffing, but i don't think they're the best for solos.
edit: on closer look at a higher quality vid,
http://www.youtube.com/watch?v=7iuv4UwDv70, jp was using a road king. some other good tones of dual rec i can think of are from nightwish's dark passion play album. emppu vuorinen's tone really cut through the thick mix but i think it's most probably due to dual or quad tracking, making the raw and harsh tone sound lush and rich.