Preamps like guitars or amps are a pretty subjective thing. I love the singing voice, especially how harmonies interweave and strengthen the subject matter. Not too much on acapella groups though.
Certain preamps 'color' the vocal in an albeit subjective way. The Avalon and Neve 1063 have been standard in megastudios since 1974 or so and were the sought after 'sound', they still are. I upgraded because some of the contemporary gospel I've been recording. At first glance, preamp discussions will lead you into thinking that they should be transparent and not add anything to the sound. The more expensive preamps are expensive because they color the sound, in a way deemed to be good.
taken/borrowed from Studio-Central
The Mic preamp is one of the most expensive components on the audio interface, which is why some do not have them, figuring you'll add your own via a mixer or by stand alone preamps. How good are they? Of course it varies. The preamps on the cheapest audio interfaces are not as good as common mixer preamps. They may lack the gain to drive some microphones, but usually work fine with common condenser mics. As you move up in the range of over $400 interfaces the preamps also improve. For example with my two interfaces, the Motu 828mk2 and the Emu 1820m, I'd rather use those preamps than those on my Mackie 1402 VLZ Pro, but it will be hard for me to tell the difference. They will all sound "very good". But if I plug in my Focusrite Voicemaster Pro preamp, there is finally some noticeable improvement. If I plug in my premium Great River preamp there is a huge sparkling difference.
For instance, Lady Monkus has a full soprano voice and can hit some really rich alto notes as well. We had a showcase at our church where members wrote original pieces and performed them. In recording the song, the firebox preamps, while adequate, didn't really 'get' the warmth of her voice. We switched a lot of mics and that didn't help much. When I got the DMP3, it added a clarity that we were looking for, it just clicked. The DMP3 imho, is the best bang for buck I've heard, and I was comfy (for a time) with the firebox preamps. Lets see how long that lasts lol !
Recording is a hobby for me but I want to do the best I can with the gear I have without compromising sound. If one day I have the extra cash to get an Avalon, or a Neve 1063, so be it. If not, it should still be ok because I'm a firm believer in tracking the best performance possible.
Hope that helps.
“Your sound is in your hands as much as anything. It’s the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use.” Stevie Ray Vaughan