UPDATE: So the FD5 came in. For now I'm just using them totally stock with the regular nozzles and the medium white "balanced" tips, into a BTR5. Nothing fancy.
In a word, dayum. Coming from the Believe, it's a definite upgrade, but since I was already acclimated to the single-DD beryllium sound, it wasn't as much of a "holy hell" moment as it might have been otherwise. I'm not going to comment too much on the hyper-technicals like decay and speed because I'm not good enough to really comment on things like that, all I can do is relate how it sounds in more "emotional" terms, so here's a handful of tracks to give my thoughts.
Tar Pond - Damn: I use this to test general "power". This is some thick, heavy doomy sludge, but it starts off with an acoustic guitar and simple vocals. The opening minute or so is nice and soft, but oh boy when the song thunders open it's like a sledgehammer. It's exactly what I wanted. The bass is heavy, powerful, but not bloated. Yes. So far so good.
Archspire - Involuntary Doppelganger: Some tech death to see if things get confused and nope not even a little bit. Being able to clearly find the bass line and each of the tom hits is pretty effing impressive. Something that goes this quickly is easy to either turn into a blur of noise or lose the heaviness for the sake of clarity. Neither a problem here.
Paysage D'Hiver - Welt aus Eis: A paper-thin black metal track to test out if it hurts. It doesn't. Actually this might be the least abrasive Paysage has sounded without losing what makes the band so damn good. For the first time I feel like I can really turn it up without it sounding artificially bottom-heavy.
Soen - Lotus: And finishing off with something proggy to let it sound musical. Not sure what to say, just that this was a very pleasant, spacious experience. Smooth as anything.
I know it sounds like I'm glowing, so let's qualify with the downsides. Single-DD, so if you need mega separation it's not here. You don't have that feeling that each instrument is being played from a different source that you can get with hybrids or multi-BA arrangements. The soundstage isn't gargantuan (even if it's damn big for an IEM). I didn't play with the tips a bunch but I can see these being too bassy for a number of people. I don't even know why they did the replaceable nozzles, honestly, especially since the thin ones ONLY come with two sets of triple-flange tips, so you can't customize the sound on that end at all.
I'll finish with this: if the Believe hadn't given me those issues with the filters, I wouldn't have bought the FD5. If I'd sampled a set after having a perfectly functioning Believe, I'd have been fairly content with sticking with what I had even though the FD5 is a clear upgrade. The Believe gets you nearly all the way there, just with a closer soundstage and a much less reliable fit (I had to readjust the Believe constantly). If you're someone with a Believe and you're uneasy with spending the extra on the FD5, I say keep the Believe and be happy. If you're waffling between the Believe and FD5 and the extra $120 isn't going to break your wallet, go for the FD5 and be happy.
I think this is right at the tipping point where spending any more stops offering as much of a benefit for the amount spent, and since I am not going over $500 for IEMs again, for the time being my journey is complete. Thanks to everyone for the help and suggestions. Time to bang my head until my neck hurts lol.
