That number is what pops up in the official Yamaha documentation for the chip that is used.
amps that are "good enough"... preamps are more linear than most, the shielding is better than most, it's a calibrated circuit with a shit ton of headroom in comparison to Hi-Fi circuits in the MkII of the UR and the Audient. These are not quite as toylike as the Focusrite Scarletts or the Behringer whatevers lol.
An op-amp is an op-amp, you don't need to switch them out, they're all pretty much the same, there are less differences between different models than between different units of the same make basically in many instances. An op-amp is made to be as close as possible to a theoretically ideal amplifier. Yeah you could go for a j-fet model and have leakage like with a tube if you like that, but it's certainly not higher fidelity than a standard mos-fet model. Whether they are discrete or not, is also a matter of application. In a strip, wher you'll want like 70 or 80 dB of gain for dynamic mics and a really low noise floor, discrete makes sense, but isn't as much of an asset as it used to be, and that application is not what is looked for here.
The outputs of the Steinberg or Audient leave nothing to be desired. There are made for powered reference monitors, have lots more headroom and a flatter impedance curve than Hi-Fi pre-amps, and they can do balanced signal transport without modifications, which in the Hi-Fi world is an ultra high end feature.
A grid powered amp with a real transformer will colour the sound like those things do, with lots of energy to spare for a much better power stage amplification than any PWM-powered or TriPath amp design could ever do, mind you, those chip-based PWM power supplies inside get high frequency switching low potential and low current power themselves from a wall wart, there is only so much they can do. The colouring of PWM-powered power amps (so-called Class D amps) and TriPath amps (so-called Class T amps) is digital, it's audible, it's more linear than an iron transformer coloured amp, but it's generally perceived as less pleasing. That is all of course a matter of taste, basically a matter of preference in the direction and the amount of colouring. Some might not want that, but might want a linear auditioning or monitoring setup. A grid-powered set of active speakers or amp can give anyone the exact custom experience he/she wants. I find it impossible to give recommendations on that level, but there are quite a lot of good options for very little money these days, from linear monitors (e.g. Fluid F5) for less than 200 bucks to an Onkyo intergrated amp with a pair of PolkAudio bookshelf speakers for a high-end Hi-Fi listening experience that lasts a lifetime. Those solutions are barely more expensive than some of the DAC/amp frivolities being sold, yet those frivolities are buspowered with 5V 500 mA or have a switching wallwart with very little to offer in terms of reliability, longevity and enough power to provide a lasting custom listening experience.
Any grid-powered integrated or power amp will have enough power to make for a far superior listening experience on any type of headphones in comparison to something that's powered by a wallwart. On most integrated amps, the headphone bus is directly powered by the end stage, just like the speakers. That also means that there is consistency in the listening experience. People who don't want that, can buy a studio grade headphone amp for very little money, and still have a headphone amplifier with real transformers in it.
Going for studio/reference is always a pretty good idea, because of multiple factors:
1. there are plugins now that you can run realtime with a perfect simulation of some of the best and most unattainable gear on the planet. You can't buy the gear that delivers this kind of colouring to the sound, but you can get exactly the same with plugins, either high resolution usual plugins or convolution ones, of which the difference with the real thing is super minimal.
2. you can always EQ to your preference. It beat having a setup that will colour to a fixed setting, because that takes away EQ-ing range, which is not the case with reference stuff.
3. if you go for reference stuff, you'll have a clean linear output wherever you use it, something that you can work with, that gives you all the options. Whether from an iPad or from a desktop PC, it'll always make it possible to get that same favorite listening experience.
Back in the day, it was the linear high quality stuff that cost a lot of money, and the non-linear crappy stuff that was cheap, but now it seems the other way around lol, because then there are the tolerances with which these DAC/amp products are often made in Shenzhen or is a garage somewhere. That is another big problem.