I own the HD 448 and I have listened to the HD 428.
They pretty much have the same frequency response as you mentioned and the soundstage is similar so the listening experience is much the same.
However there is a noticeable difference in highs and mids, the HD 448 sounds very neutral where the HD 428 sounded a bit distorted in this section (nothing to worrying though).
The HD 428 basically sounds cheaper but my ears are used to the HD 448 so the comparison is not entirely fair.
Moreover, I am not 100% sure the HD 428 was burned-in but in my opinion you better just spend the $30 so you don't have to doubt you have got the best sounding headphone.
This is because the HD 448 truly sounds amazing (if you don't need a lot of bass) and I certainly recommend it if you want very detailed, natural sound.