i’m confused, what’s a pre-amp for anyway?
Preamps control the voltage of the input signal to the amplifier circuit.
background: currently deciding between magni 3, heresy and atom amp to drive my incoming 58x’s.
Those are headphone amplifiers. Headphones just have built in preamps to control the voltage into their amplification circuits unlike speaker systems that can have separate preamps and power amps. Some headphone amps can have an output that bypasses its own amplification circuit designed to drive headphones and send the signal to a speaker power amp though.
I want to run the source via a lightning to rca (for iphone) straight to the amp for simplicity sake but wondering how the pre-amp figures into all this.
The preamp will take an input voltage from a source unit like a CDP/music server or either with a DAC or a TT with built in or outboard phono preamp, then either passively reduce the 2V to 4V signal to control the output, or use an active circuit to boost that signal depending on how far you twist the dial. That preamped signal enters the amplification circuit, whether in the same device or another, where it gets amplified to a stronger signal that can drive a transducer (ie for simplicity's sake this has current delivery where a preamp only mitigates or boosts voltage).
Those devices you listed have a preamp circuit and headphone output stage circuit built in so instead of using the battery of the iPhone to power an integrated chip inside the dongle with about 15mW at 32ohms you use a headphone amp with its own power supply that can kick out somewhere along the lines of 3W into 32ohms or 300mW into 300ohms, and preferably at lower noise and distortion levels...assuming the signal that enters its preamp circuit is very clean ie no noise (all it can guarantee is possibly not introducing too much of its own distortion). This allows you to go louder without the sound breaking up, again assuming you're feeding it a clean signal.
Think of the iPhone and dongle as any mobile APU, a headphone like that as an X86 game/software that doesn't run on Rosetta, so now you get a an AMD 5950X or Threadripper 3970X and AMD Radeon 6800XT to run that game. Alternately the iPhone is like a Prius powertrain, the headphone as a BMW M5 chassis, so the headphone amp is like getting a BMW M5 engine; the dongle or any protocol you use to get the audio from the iPhone to the amp is kind of like fuel and injectors, which can range from "needs a shot of STP" all the way to "aviation-grade fuel used by people with high PSI forced induction engines."
