Man! That sure is a tricky question. This is mainly because most earbuds you get today are either warm and have a really good bass presence or are boosted in the ear gain region to where they skew what might otherwise sound like the Sennys. As for Koss, I have never been a big fan of their head gear (except those free ones you used to get with mp3 players, you know the ones that go on your ears and wrap around the back of your neck?).So... I'm enjoying the FF3 way more than I thought I would. This is my first earbud costing more than a Monk+...which I also enjoy way more than I thought I would! Are there any Koss fans out there? In your estimation, what are the earbud parallels to both the Porta Pro and KSC75 (w/headbands)? And what is the earbud parallels to the HD650 and HD600? I hope that question makes sense... I'm trying to transition and get my bearing in the earbud world by referencing some well-known headphones.
None of the buds I have, have many similarities to the HD650, or the HD600 other than being an audiophile (potentially) neutral listening experience. But I don't know of any that have that "house" Senny veil, and neutral bass (at the same time). The FF3 might come closest to what you are asking (except the boosted bass). The FF5 have much more air in the treble compared to the FF3. The Yinman 2.0 600 are absolutely fantastic, but they have some big bass (compared to the FF3 they are more sub bass focused and less punch), and I would call them warmer overall in tone than at least the HD600.
Having said that, the Yincrow X6 might come close (though they don't have near the imaging or separation that you would get from the HDs, just similar tuning except I think even these have more quantity of bass overall IIRC).
Keep in mind, I don't own either of those anymore, and am going by memory. I have the HD700, but it is more akin (or in between) the HD650 and the HD800.
So basically, I spent all this time typing just to tell you that I am not much help for you... LOL This is where normally our resident "has heard every bud since the dawn of buds" (@WoodyLuvr) would be able to chime in an tell you what you want to hear, but he very rarely makes an appearance on this thread nowadays.