With the given experience you can distinguish different (real) orchestras, conductors, grand pianos, guitars and maybe even their pickups so, why not sample libraries?
The mics & their position plays a bigger part on any instrument than you might think. When micing a guitar cabinet for example, the end result will vary much greater moving a mic even 1cm to any direction, than changing the amplifier to another & having the same mic & position (talking about average, not extremes). Mics also sound very different, not to mention the space they are recorded in, and in this example, the cabinets & speakers too. Then there's the instrument itself, and often overlooked; the player(s). There are endless tonal possibilities, even with the exact same instrument & notes, so every sound library has a distinctive sound for this reason. If you want every grain that's a part of your masterpiece to be totally unique sounding, without anyone knowing the source, you should avoid using presets/samples as is, but as a gateway into your own realm of soundscapes, where you only begin to apply various FX to mold your sonic world as you imagine it. If the sounds themselves are just to serve a small fraction (although important) of your composition, where it doesn't matter that much whether some strings come from an old Casio (can sound great with FX!) or a big sampled library, you shouldn't care if someone knows what vst/source was used. Average listener wouldn't know & wouldn't care. They're interested in the world you've opened for them, not how you built it. They're hungry for new soundwaves entering their souls like morphine, which is more the reason to avoid using straight presets/samples as is, if you want to be their new found hero, that is. If you don't want your sounds to be connected to any vst/library for whatever reason, look into layering different sounds together & don't be shy letting FX take you on a journey to the unknown. Experiment & allow yourself to get lost, no matter where you'll wind up, it will build up on your musical knowledge. As mentioned, even when using plain presets/samples, there are still endless tonal possibilities to carve 'em like a boss.
