Battle of the basshead buds:
Yincrow X6, Fiio FF3 and Destiny
I'll start this off by saying that this won't be a super in depth comparison, I'm not much of a critical listener and my methodology isn't the most thorough. These impressions are pretty vibes-based. I'm also one of those people that aren't particularly sensitive to differences in staging, I've never noticed much of a difference between transducers of the same type, so I won't mention it here.
First up: Yincrow X6
There's a good reason that these are the most common entry point for those looking get into flathead earbuds: they have a good amount of bass, decent technical chops and a sense of space that's distinctive of flatheads.
Having said that, when compared to the other two it's clear that the bass is more mid-focused and doesn't extend that deep. The treble is also slightly grainy (to me) and rolled off, making it feel veiled when compared to the other two.
Fiio FF3
One of my favourite buds, they have great, authoratative bass that reaches down into the sub-bass regions (around 45Hz). Great for any electronic music (which is most of my music library) . They lively sound the most of the three and are the most technical, though not by a whole lot.
They're also fairly heavy. After a few hours I need to take them out and give my ears a rest, which isn't as much of an issue as the other two which are much lighter.
Destiny (DIY bud by @mt877)
I built these myself using the brighter tuning and standard foams. I don't have any Heigis that are recommended but for the purposes of this comparison I don't think it would make a difference. I also found installing the paper filters extremely fiddly even with tweezers so I can't guarantee they sound exactly the same as everyone else's.
The biggest pro with these is that the bass is very similar to that of the FF3. It digs almost as deep and has almost as much authority. It's also more forward with female vocals which is makes it feel a bit more well rounded.
However, the treble isn't quite there making it feel a little flat when going head-to-head with the FF3.
Conclusion:
If you're looking for a basshead bud, the FF3 is the clear winner here. However, they're discontinued and getting fairly expensive. Building a Destiny will get you 95% of the way there and a possible upgrade in comfort if you're happy to accept a slightly more laid-back sound. It's also a fraction of the cost for maybe 20 minutes of work assembling.