Well, that the SahsrAra Lotus is the abode of Lord Shiva-SadAshiva-Parashiva is mentioned in numerous Tantras and Agamas. And, by the Abode of Shiva, we only mean KailAsha.
I have the KulArnava Tantram book. While, commenting on 5.107 (which talks about the Shakti Pithas besides other things) of the aforementioned Text, the editor/translator quotes from YogAsAra Tantram as follows:
According to YogasAra Tantram MulAdhAra is KAmarupa Pitha, AnAhata
Chakra is Purnagiri Pitha, Visuddhachakra is JAlandhara, AjnA Chakra
is UddyAkhyA or the UddiyAna Pitha and SahasrAra is KailAsha.
In JyAna BhAshya Tantram, Lord Shiva discusses about Shatchakra Nirupuanam (determination of the six-chakras). While discussing the Ajna Chakra he says:
AjnAnAma bhruvor madhye chakrancha dvidal param |
samyuktam nirmalam sumanoharam ||
ItarAkhyAm mahAlingam tanmadhye
AjnA samkramanam tatra gurorAjyeti vishrutam ||
KailAsAkhyAm tadurdhe tu vodhini tu tadurdhatah |
Between the eyebrows, there is two-petalled Supreme Chakra attached with the
two letters Ha and Ksha. Within that Chakra lies the Great Linga called
Itara whose brilliance matches that of gold. Here the transmission of
Guru's orders (AjnA) takes place, hence the name. Above this Chakra
lies the KailAsha (the 2nd Bindu), and above KailAsha lies the Vodhini
Commentary on Vodhni Shakti is as follows:
Above Ajna Chakra lies Shiva-Swarupa 2nd Bindu (KailAsha). Above which
lies half-mAtrA-kArA (unable to translate mAtrA) Vodhni Shakti; Above
which lies the half-moon (crescent)-shaped NAda which is Shiva-Shakti
combined. Above that is present AjnirupA VyApikA Shakti; above that
Samani and even above lies the Unmani- These seven KAran Rupa are
present in that order. Bindu, Vodhini, NAda--- These three are forms
Then, while discussing the SahsrAra Chakra, he says:
SahasrAram mahApadmam nAdabindu samnvitam |
Shunya rupah Shivah
sAkshAd vrittam parama kundali ||
SArddha trivalayAkArA koti
vidyut samaprabhA ||
[Above Vodhini] exists the Great SahsrAra Lotus, which is comprised of NAda-Bindu.
The void in this place is SAkshad Shiva (Shiva Himself) and the circle (Vritta) is
Supreme MahAkundali, which embraces [Shiva] in three-and-half circles and which
shines with an electrifying brilliance.
So, while the first few verses say that KailAsha is above AjnA, and the second few verses say that SahsrAra is also the abode of Shiva. So, there can be two KailAsha, Subtle-Gross or something like that.
Now, in YAmala Tantram too, Lord Shiva explains this Circle and the Void further:
Vrittam kundalini shaktir gunatraya samanvitA |
mahAdevi shivrupo maheswaraha ||
SarpakArA shivam veshtya sarvadA
tatra samstithA ||
Shiva shakyAtmako bindur bhukti mukti
Hey MahAdevi- The Circle is the MahAkundalini Shakti which comprises
the three Gunas. And the Void [in the thousand-petalled Lotus of the
SahsrAra] is Shiva-Maheswara Himself. The snake-form Kundalini,
embracing Shiva, is always present at this place. The Bindu, which is
verily of the form of Shiva-Shakti, is the giver of both worldly pleasures and liberation
. That Eternal Kundalini Devi when unites with the NAdarupa Shiva
(Shiva who is of the form of NAda, Sound) becomes the Prakriti.
So, without doubt the SahsrAra is the abode of Lord Shiva.
The next block quote is from the Gandharva MAlikA Tantram, which explicitly mentions the abode of Shiva as the SahsrAra. Here the verses describe not only the Abode briefly but also tell us how to meditate upon Shiva (or SadAshiva) who presides over this Abode.
SahasrAram shivapuram ramyam dukkha vivarjitam |
kalpavriksham sushobhitam |
Pancha bhutAtmakam taccha guna traya
Chaturveda chatuh shAkham nityapushpa phalAnvitam |
The SahsrAra Lotus is the abode of Shiva. That place is pleasant,
beautiful, devoid of sadness and is filled with beautiful
fruits-flowers-full trees. The abode of SadAshiva is supremely
pleasant. Here is present the Kalpavriksha which is of the form of the
Trigunas as well as the form of the Pancha MahAbhutAs ( five
elements; earth, fire etc). The four Vedas form the four branches of the Kalpa-Tree.
In the next few verses from the same Tantra, Lord Shiva teaches how to meditate upon Shiva who lives in the SahsrAra:
DhyAyet sadAshivam devam suddha sphatika sannibham ||
samAkirnam dirghabAhum manoharam |
Sukha prasanna nayanam smerAsyam
satatam priye ||
shravane kundalopetam ratna hArena shobhitam | ....
Ashtabahum trinayanam shivam padma dale kshanam |
ramyam shabda brahma mayam vapuh ||
Evam sthuula pustasya bhAvayet
Meditate upon the pure and the crystal-like-white SadAshiva thus- He is
decked with ornaments made of varied and many precious stones. His
arms are long and attractive. His eyes are happy and they are shining with bliss. On his face is present a pleasant smile. In ears he is wearing
Kundalas. The body (neck) is decked with a precious stone-made
necklace as well as with a garland which is made of thousand lotuses. His arms
are eight in number; His three eyes are wide resembling the
lotus-petals. The legs are decked with beautiful Nupuras. His body is of
the form of Shabda-Brahman- Hey lotus-eyed one! Shiva and his gross
body is to be meditated upon thus.
In Shakti MAheswra Tantram, Lord Shiva teaches Kundalini Yoga. And in that context he teaches how to move the MahAkundali upwards towards ShasrAra where lives SadAshiva.
Gacchanti brahmarandhrena bhittA granthim chaturdasha |
sandhi mArgena sushumnA vartmanA tathA ||
Hamsena manunA devim
sahsrAram samAnayet ||
SadAshivo mahAdevo yatrAste parameswari
Tatra gatvA mahAdevi Kundali paradevatA |...
Bring the Kundalini-Shakti, which is travelling up towards the
Brahmarandhra, through the Sushumna channel, by piercing the 14
Granthis (14 Knots as explained below), by employing the Hamsa Mantra, to
the SahsrAra Lotus. Where MahAdeva-SadAshiva resides...
Commentary on the 14 Granthis or Knots which the Yogi has to pierce through his SAdhana:
14 Knots- Three lingas (Swayambhu, BAna, Itara); ShatChakra; Pancha
Shiva viz: BrahmA, Vishnu, Rudra, Iswara and SadAshiva)
Reason for the update is the fact the question was updated.
Well, what the Avadoota Yogi has realized by his practice is also supported in the scriptures.
For example, Lord Shiva says:
Shatchakram parameshAni sadAshivapuram samam |
maheshAni sadAshivapuropari ||
The Shatchakra is the abode of SadAshiva. Above that SadAshivapura
lies a similar Shaktipura-the Abode of Shakti.
So, what this verse says is that Lord SadAshiva presides over the Shatchakra. His abode itself is the Shatchakra. And, above that lies the Shaktipuram or the Abode of Shakti. Note that its also said that Shaktipura is similar to SadAshivapura.
Also, although Shat refers to six, by Shatchakra all the seven Chakras (from MulAdhAra to SahasrAra) are always implied, with the SahasrAra as the chief and controller of the lower Chakras.
So, upto SahasrAra is the Abode of Shiva, and above that lies the Abode of Shakti. I am not sure though which scriptures discuss the details of this Shaktipura.
Note: Since i am following all Sanskrit-Bengali books hence i sometimes find translating some of the words in the texts, in English troublesome. The Bengali translations, more often than not, simply leave the original Sanskrit in them since the same words are also used in Bengali. So, in order to find the most appropriate English words i have to consult a dictionary which becomes time consuming. That's why in my answers i often use Sanskrit as it is sometimes.