In order to answer this question, you need to be able to distinguish between music and song. Music is something that uses musical instruments to create tones, beats and the like. Song on the other hand is vocal, just a human speaking with his own voice, without the use of musical instruments. However songs can also include music as that is the norm these days.
Musical instruments of all kinds are prohibited except the Duff. Duff is a one sided drum. Flat on one side and open on the other. All other musical instruments are Haram.
The Messenger of Allaah (peace and blessings of Allaah be upon him) said:
-“Among my ummah there will certainly be people who permit zinaa, silk, alcohol and musical instruments…” (Narrated by al-Bukhaari-ta’leeqan, no. 5590; narrated as-mawsool-by al-Tabaraani and al-Bayhaqi. See-al-Silsilah al-Saheehah-by al-Albaani, 91).
So this proves why all musical instruments are Haram.
Hadith
حَدَّثَنَا عَمْرُو بْنُ رَافِعٍ، حَدَّثَنَا هُشَيْمٌ، عَنْ أَبِي بَلْجٍ، عَنْ مُحَمَّدِ بْنِ حَاطِبٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ
فَصْلُ مَا بَيْنَ الْحَلاَلِ وَالْحَرَامِ الدُّفُّ وَالصَّوْتُ فِي النِّكَاحِ "
It was narrated from Muhammad bin Hatib that the Messenger of Allah said:
"What differentiates between the lawful and the unlawful is (beating) the Daff and raising the voices (in song) at the time of marriage."
Hasan (Darussalam)
English : Vol. 3, Book 9, Hadith 1896
Arabic : Book 9, Hadith 1971
Get Hadith Collection (All in one) App:
https://goo.gl/8j06i9
This Hadith explains that Duff is allowed as a musical instrument. Nothing else.
As for singing, the Prophet PBUH said: The one who does not sing the Quran is not one of us.
Here sing means to recite in a beautiful manner.
So songs, if the lyrics are have nothing Haram in it, and you sing it without the use of musical instruments, then it is allowed, you may sing.
Then you said, some of the best ever musicians are from Islam. Well, that has nothing to do with Islam. It is their problem. Did Islam tell them to sing? Not at all. So Islam is not responsible for their deeds. And I don't know what you mean by they are pious and religious, just because someone prays or does Hijab doesn't have anything to do with piety. Piety comes from the Heart, not from external appearances. Of course they will pray, otherwise it is an act of disbelief. So just because a Muslim prays, doesn't mean he is very religious. It's just his basic duty. So calling people religious just by seeing a few good things is not the job of a Muslim. Also, they earn by singing and use of musical instruments, how can someone be doing Haram for a living and then you call them pious and religious? I don't know. Maybe they are but we are not the one to judge whether they are or they are not. Leave it to Allah. But no matter what, even if an Islamic scholar does Haram, then it is Haram. So if those people who you mentioned, sing, then it is Haram, no matter how much religious you think they are.
Hadith
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صلى الله عليه و سلم إنَّ اللَّهَ طَيِّبٌ لَا يَقْبَلُ إلَّا طَيِّبًا، وَإِنَّ اللَّهَ أَمَرَ الْمُؤْمِنِينَ بِمَا أَمَرَ بِهِ الْمُرْسَلِينَ فَقَالَ تَعَالَى: "يَا أَيُّهَا الرُّسُلُ كُلُوا مِنْ الطَّيِّبَاتِ وَاعْمَلُوا صَالِحًا"، وَقَالَ تَعَالَى: "يَا أَيُّهَا الَّذِينَ آمَنُوا كُلُوا مِنْ طَيِّبَاتِ مَا رَزَقْنَاكُمْ" ثُمَّ ذَكَرَ الرَّجُلَ يُطِيلُ السَّفَرَ أَشْعَثَ أَغْبَرَ يَمُدُّ يَدَيْهِ إلَى السَّمَاءِ: يَا رَبِّ! يَا رَبِّ! وَمَطْعَمُهُ حَرَامٌ، وَمَشْرَبُهُ حَرَامٌ، وَمَلْبَسُهُ حَرَامٌ، وَغُذِّيَ بِالْحَرَامِ، فَأَنَّى يُسْتَجَابُ لَهُ؟". [رَوَاهُ مُسْلِمٌ]"
On the authority of Abu Hurayrah (ra):
The Messenger of Allah (ﷺ) said, "Allah the Almighty is Good and accepts only that which is good. And verily Allah has commanded the believers to do that which He has commanded the Messengers. So the Almighty has said: "O (you) Messengers! Eat of the tayyibat [all kinds of halal (legal) foods], and perform righteous deeds." [23:51] and the Almighty has said: "O you who believe! Eat of the lawful things that We have provided you." [2:172]" Then he (ﷺ) mentioned [the case] of a man who, having journeyed far, is disheveled and dusty, and who spreads out his hands to the sky saying "O Lord! O Lord!," while his food is haram (unlawful), his drink is haram, his clothing is haram, and he has been nourished with haram, so how can [his supplication] be answered?
[Muslim]
Sahih
40 Hadith Nawawi 10
English translation : Hadith 10
Get Hadith Collection (All in one) App:
https://goo.gl/8j06i9
So if a person earns Haram, is nourished with Haram, and so on, then it is very difficult for them to ask help from Allah, because Allah does not accept their Dua and Ibadah. So how come you say they are so religious?
And finally, Sufism is a deviated sect from the Sunnah of the Prophet, and their Songs almost all include Bidah and Shirk. So those are totally Haram even if you don't use musical instruments, because the lyrics contain Shirk, a whole lot of them.
So listen to the Quran instead, and don't run towards what some people tend to make you believe is Halal. They are not.
Jazakallah Khair.