Sometimes external dacs do sound identical to built in dacs (to a particular person on a particular system). My first two dacs fell under this category. The first, an Audio Alchemy Dacman (BOTL), purchased used in 96 so that I could play music from my computer to my stereo via my Opcode Datport (USB<=>Coax converter box that I used to import DAT to computer) sounded identical to the built in DAC of a Sony 60ES DAT, Fostex D5 DAT, Tascam DAP1 portable DAT, and Sony CA8ES carousel CD changer. It bested the built in DAC of the Sony D7/8 portable DATs (same basically). As far as all the units that sounded the same, I tried and tried to discern subtle differences but it was tough and I concluded they all sounded the same to my ears. Having said that, I certainly think it is very probable that others, with better systems and hearing could. Maybe I could on a better system. That would not surprise me. I think my hearing is good/very good/maybe better but it is probably not as good as I think it is and I think there are many with much better hearing (and much, much better systems) than me.
Not exactly thrilled with this result (perhaps I should say I was thrilled with how good the built in dacs sounded, its not that I ever thought they sounded anything but great, but I hadnt compared them to anything), I purchased a newer, and I believe slightly higher end dac a year or two later (new, but on blowout for $100), a Musical Fidelity X-ACT 18 bit. It had been replaced by a “24” bit dac, the 24k?, another cylinder. Same thing, though if I had to pick one of the two, Id say the earlier Dacman (at less than 1/10th the volume of the cylinder-on-its-side X-ACT) sounded ever so slightly better. But that is splitting hairs, for all practical purposes they sounded the same. To me on my system.
I havent had/listened to those dacs since 01 and didnt own another until I bought the B&K AVP-1030 5.1 pre (1996 model) in 2011. The dac in this is clearly better than the earlier stuff, though for various reasons (sold Fostex to friend, CA8ES died while sitting in a box, Ben Harper 5-17-97 front row center master stuck in 60ES, wore out D7/8) I could only compare it to the Tascam DA-P1. The B&K (and Topping D30 with stock wall wart) is next level and change compared to the earlier stuff, though its not much newer. Actually its from the same few years (94-96) as the 60ES, D5, CA8ES, DA-P1 and D7/8. It was a significantly pricier unit though, $2500. The 60ES, D5, and DA-P1 were about half that, and the CA8ES and D7/8 about a third.
I suspect Id be extremely impressed, perhaps even blown away by a DAC such as the D70 or higher end. I wonder if a D90 is in the works?