frozenOx
100+ Head-Fier
Keep an eye on the AVExchange subreddit. Sundara's routinely come up for ~$280 but they typically sell the day they're posted.
The 400i do well with metal. They have a bit of treble emphasis but are not what I would describe as bright by any means. Not a ton of sub bass and bass impact but what's there is fast and punchy. They take EQ well if you don't mind going that route. For the price, I think they're a great option. Open box from HiFiMan at $139 is a steal IMO.
I recently received the NF Audio NM2+ and I'm not sure I'll keep them. I need to give them some more listening time but they're very bright and fatiguing with black metal, most death metal I've listened to or really any compressed mix. They're very detailed, and have great bass response, but they do not shine at all with the metal I typically listen to. Interested to hear how you like the Believes. Those are on my IEM wish list.
That's good to know about the NM2+. I don't think I'll be ordering anything from Aliexpress anymore. My IEMs are bouncing around "partner facilities" for the past week and haven't even been given to the USPS yet.