This is wrong on so many counts. Firstly - 'Professional' can be applied to level of equipment, or natural or acquired ability, or mainly nowadays, the ability to generate a profit. The real word we should be taking about is not professional vs amateur , but not being amateurish - which is very different. An industry pro could make an excellent recording with an SM58. Not a cost issue, not a quality issue, but just using the right tool for the right job.
Monitoring is personal, but there are variables. Open headphones or monitor loudspeakers allow your pair of ears and your brain to work together. Some people might get their track to sing to from a monitors or open backed headphone and hear enough of their own voice to perform. When some people sing loudly, they need to have the monitors turned up loud and they spill. So for recording, headphones are essential. If, however, you use sealed headphones or in ear monitors, most people absolutely need some of their own voice. How much depends on what they need to be able to sing properly. This applies as much to opera singers as it does for metal merchants. For some people, not having their own voice at exactly the right volume in sealed headphones/IEMs means they sing out of tune - sometimes exactly a certain degree flat or sharp. Remember folk singers who used to put a hand over one of their ears? They did it to change what they can hear. Loads of bands now use personal monitor mixers so they can have individual control over their mix - my band do, and to get the best performance I NEED, not want, the right things in my IEMs, and my mix is different to the others. I want lots of me, lots of my bass, then 75% the guy who sings the high parts, then the guitarist at about 50%, the drummer I have really low at 25% because he puts me off. I have NO guitar, because it's level is high and close and enough leaks, and I have the drum overhead - but the keys are vital so they're around 80% ish. If I cannot get this because something goes wrong - I'll pull my IEMs out and manage on spill from the stage sound. If I cannot hear my voice, I cannot sing in tune - for me, it is impossible. The person who sold you an audio card should keep to selling electronics and not tell you the biggest pile of poo I have heard for years. You need what YOU need. Nobody else. If you sing better with no voice - that's cool. If you sing better with you scarily loud, that's good too. Remember the people who are big names. They are ONLY interested in their own personal mix. What the audience hear is somebody else's job.
Professionals realise that to play your best, you need certain things, and we are all different.
