I am not a fan of 3.5mm TRS connectors on the headphone/driver connection for any headphone. 2.5mm is worse. But neither is “good” in my opinion. Even when it’s properly supported, doesn’t limit your choice of connectors unduly, and has some kind of locking mechanism.
However, here’s what you need to know about the RAD-0 headphone connections.
First, yes, they’re 3.5mm TRS.
The outer barrel of that connector is 7.62mm (0.3") in diameter. The recess/hole in the driver cup that accepts it is 8.0mm (0.314")*. The full length of the TRS conductor piece is 15mm, and an additional 4mm of the connector’s barrel inserts into the socket before it is fully seated.
Standard 3.5mm TRS plugs from Furutech, Neutrik/Rean, Eidolic and so on have the same 15mm TRS plug, with up to a 2mm long base that fits into the socket on the RAD-0, but unfortunately this leaves them 1.5mm to 3mm shy of full engagements before the barrel of said connector blocks any deeper insertion.
The only connectors I have found, so far, that will engage properly are the extended sleeve 3.5mm TRS plugs from Eidolic, which are often sold on cables for certain HiFi-MAN headphones:
While these fully engaged, the extended sleeve is only 5.5mm in diameter, so leaves 2.5mm of space and isn’t supported by the recesss in the RAD-0’s socket. In addition, the extended sleeve extended beyond the socket by about 5mm:
Pinouts will follow - I have them, just not where I’m sitting while posting this.
Personally I’d have much preferred these were mini 4-pin XLR with the same pinout as Audeze and ZMF, or mini 3-pin XLR with the same pinout as JPS Labs. It’s not like the extra gram or so would have had any significant impact on the already prodigious weight of the things, and then locking mechanisms would be standard and there’s several excellent options for connectors, with the bonus that they’re much easier to solder and secure than TRS plugs.
I suppose if one wanted to debase ones warranty you could swap the connectors and/or, since the shell is a resin/acrylic, and fairly tough seeming, you could probably drill out the recess to be a littler wider without much problem (can’t say if the material used is prone to splitting, but just a 1mm increase in diameter would let you use the same cables/connectors that, say, all the Focal headphones (Utopia excepted) use …
*Interesting that 7.62mm is a NATO spec bullet caliber, and that 0.314" is π.