Tonal difference between the "REISSUE" API 550a vs 550b?

Not much of a help since I done own both but I have 550A’s and 560’s. Go to Zenpro audio’s clipilator and you can listen to both the 550a and 550b on a snare drum. To me there is something happening in the 550a circuit or the curve more likely that I really like compared to my experience in the past with 550b’s and the more recent clips I’ve heard. 550’s are great for more fleixabilty. But to me if you’re tracking and what to just sweeten up a source with some addictive EQ the 550A’s get the nod in my book.

other than the bandpass filter and 3 vs 4 bands, from my understanding the current issue 550a and b are the same circuit. same sound, curves etc.

right, the 2520 and the 550A itself have evolved. but current issue As and Bs should sound the same as each other.

Actually your post was quite helpful, thanks.  I went to the Zenpro site and listened to all the clips for the 550a & 550b.  Not sure how much this translates accurately through the web but I didn't really hear a difference between the two.  Obviously that's different than having the units in front of me and using them on a track, but I have a feeling that I would have similar observations 

smashlord , I have noticed this difference  you mentioned in some of the 550 plugins by Lindell and Waves -  not sure if that's because these plugins were emulating vintage units.  I'm assuming though that you're also talking about the newer 550's as well.

My primary use will be for electric distorted guitars and also for clean electric & acoustic.  Will also probably use them on Snare.

Yes, talking about newer units. I know some prefer the sound of the older 550As, but the newer units give you the 2.5khz and 600hz bands (among others) that I personally use the shit out of. Functionality> last 5% in tone (if it truly exists).

If your primary use is electric guitars, especially distorted ones, I might recommend the 550B. As I mentioned, it doesn't smear things like the 550A does, which is useful on something that is already smeared, like a a distorted guitar. Its EQ points are also more useful for guitars and boosting the fundamental of different snares with different tunings. I typically choose the 550A on snare only because it does a cool thing to the transient, but it usually requires some further shaping with another EQ, where as the 550B could probably get there on its own.

One other thing to remember and Paul Wolf has touched on this in a couple of interviews is the transformers have evolved over time in both sourcing and construction.

Steiger and I have had private discussions about this: Saul Walker wasn’t as much a design genius ala Rupert Neve, his Genius is more in line with Mr Peavey, he sourced some already available components, refined basic designs ( Current API will argue they have the rights to the API knob, but it was sourced from the catalog of an OEM, they do hold a patent on a dual concentric but they were buying dual concentric knobs from the OEM decades before the patent was filed).