The A6 is pretty much its own beast, with its own sound. Sure, it can be pressed into duty in emulating the sounds of a JP-8, or OB-whatever, or Prophet, or Moog, but it'll never nail them exactly, any more than you would expect a Roland synth to nail an Oberheim sound. It can get pretty close though, mainly since Alesis used a number of classic analog designs when planning the A6. The VCOs and 24db filter are based on Moog Modular, 12db filter on Oberheim SEM, the ribbon controller is like that on a CS80, etc. To do everything the A6 can do on vintage synths, you'd need a Don Solaris-sized collection of them!
So it's really its own instrument, with its own sound, just like any other synth. The A6 has its sound, the OB-X has its sound, the JP-8 has its sound, the Minimoog, the ARP Odyssey, etc. All are unique.
I do have to say though, one thing the A6 does well, and I love this about it, is it can do the Vangelis Bladerunner
brass sound really well, and programmed right, it can be very expressive.