Of all the decks I'd heard over the years, the Harman Kardons were the only ones I'd ever heard that IMHO had sonics that would give the Naks a run for their money. Great performance at a lower price. Their distinguishing characteristic was that they used "ultrawideband" electronics (the response of the electronics was well beyond our range of hearing), which cost more but prevented some of the distortion and sibilance that cheaper decks exhibited.
My first H/K was flaky, but, they exchanged it for a CD301 and I've made tapes on that deck using the lowly TDK D-90 that amazed others with how good they sounded. 20-20kHz response with Type 1 tapes; 20-22kHz with Type 4 (metal). I've always wanted the CD491 as that was the pinnacle of their cassette decks IMHO, and they seemed to cheapen up with the later models, as I owned one of those also and it was no different from the average decks I had quickly heard and disposed of.
Never had an issue with Dolby either--having the alignment capabilities of both the bias and Dolby levels made for some really nice listening and none of the artifacts I'd hear in other decks. Since I wasn't a cassette snob, I did not want to spend sick money on a good Nak ($1000+)--I just wanted something that came closest to what I heard in my audio system, for portable use. (Either in the car, or on a Walkman.) I'm done with cassettes now of course, but I have to say owning the H/K was a pleasurable experience all around for me.
That's just my experience; your mileage may vary.
BTW, dual capstan is worth it. I think Nakamichi used to push the pressure pad away from the cassette, and have the front capstan turn just a hair faster than the rear capstan, which kept a constant pressure of the tape against the tape head. I don't even know if the H/K does this, but, it would be ideal no matter who offers it.