So, I found this book titled “Practical Meditation Guide By A Forest Monk by The
Reverend Brahmadeva Forest Monk”. It is a free book. The site of the book  is rather sketchy but if you ignore it and read the book the content is quite interesting.
The book says that the arupa brahmas are not sentient, i.e. they don’t perceive things. AFAIK there is also no mention of a being from the arupa loka that come to visit the Buddha in the tipitaka. While there are mentions about brahmas from the rupa loka (e.g. Brahma Sahampati) that visited the Buddha.
What more can you tell about formless brahmas? Formless brahmas
are also beings. But they are not sentient beings. Their minds are fixed
onto the formless meditative objective by birth. They are unaware of
anything else. Unconscious brahmas are also beings. But they also are
not sentient beings. Their minds too are fixed onto the meditative
objective by birth. Therefore, they too are unaware about anything
else. Other than these creatures; all other creature types are sentient
beings. (p. 1065).
This also means that arupa brahmas cannot practice insight meditation and they cannot gain enlightment while in the arupa loka.
 https:// practicalmeditationguide .mystrikingly.com/
Im not sure if thats true. I thought it was only brahma beings in the unconscious realm ( asaññasatta) which are essentially not sentient.
Arupa brahmas don’t have the body/Rupa. They only have the mind. That is why they are called arupa. So they don’t have eye and ear. That is why they can’t come to meet the monks or listen Dhamma.
Asanna brahmas don’t have the mind/Sanna. They only have the body. That’s why they are called asanna. So they can’t do anything.
Let’s say that is true, but those who have attained jhanas and reborn in the brahma world have supernormal powers, so even if they don’t have a body they can create a body. There are also dibhacakkhu (divine eye) and dibbasotta (divine ear) which means in jhana, sight and hearing are not limited by phyiscal organs.
I’d have to look this up, but …
It is said to be dangerous to be born in an arūpa plane of existence , or the special 4th jhāna realm (Asanna brahmas) because they can miss the teaching of the Buddha.
One of the big flags of Ajhan Mun’s book is he is communicating with a female which I guess is an arūpa jhāna being who I think is in love with him. (It has been a while since I read some of this book and I really don’t want to read more… but I will search for that exact quote).
This story concerns Ãcariya Mun’s longtime spiritual partner.17
Ãcariya Mun said that in previous lives he and his spiritual partner
had both made a solemn vow to work together toward the attain-
ment of Buddhahood. During the years prior to his ﬁnal attainment,
she occasionally came to visit him while he was in samãdhi. On those
occasions, he gave her a brief Dhamma talk, then sent her away. She
always appeared to him as a disembodied consciousness. Unlike beings
from most realms of existence, she had no discernible form. When he
inquired about her formless state, she replied that she was so worried
about him she had not yet decided to take up existence in any speciﬁc
realm. She feared that he would forget their relationship – their mutual
resolve to attain Buddhahood in the future. So out of concern, and a
sense of disappointment, she felt compelled to come and check on him
from time to time. Ãcariya Mun told her then that he had already given
up that vow, resolving instead to practice for Nibbãna in this lifetime.
He had no wish to be born again, which was equivalent to carrying all
the misery he had suffered in past lives indeﬁnitely into the future.
Although she had never revealed her feelings, she remained wor-
ried about their relationship, and her longing for him never waned. So
once in a long while she paid him a visit. But on this occasion, it was
Ãcariya Mun who thought of her, being concerned about her plight,
since they had gone through so many hardships together in previous
lives. Contemplating this affair after his attainment, it occurred to him
that he would like to meet her so they could reach a new understanding.
He wanted to explain matters to her, and thus remove any lingering
doubts or anxieties regarding their former partnership. Late that very
night and soon after this thought occurred to him, his spiritual partner
arrived in her familiar formless state.
I heard that was the case with brahma beings in general. since thier lifespan is so long. Someone told me the Buddha recommended going no farther than the 6 deva realms to avoid this, tho not sure how true that is. Of course the Buddha can still teach brahmas, except for asanna and maybe arupa brahmas, so its not as much of a loss, even if it is not ideal to be a brahma while the Supreme Teaching is alive and well.
I think you can see from the Dhamma Cakka Pavattana Sutta that the sounds went all the way up to the Brahma Loka.
pavattite ca pana bhagavatā dhammacakke bhummā devā saddamanussāvesuṃ – ‘‘etaṃ bhagavatā bārāṇasiyaṃ isipatane migadāye anuttaraṃ dhammacakkaṃ pavattitaṃ appaṭivattiyaṃ samaṇena vā brāhmaṇena vā devena vā mārena vā brahmunā vā kenaci vā lokasmi’’nti. bhummānaṃ devānaṃ saddaṃ sutvā cātumahārājikā devā saddamanussāvesuṃ – ‘‘etaṃ bhagavatā bārāṇasiyaṃ isipatane migadāye anuttaraṃ dhammacakkaṃ pavattitaṃ, appaṭivattiyaṃ samaṇena vā brāhmaṇena vā devena vā mārena vā brahmunā vā kenaci vā lokasmi’’nti. Cātumahārājikānaṃ devānaṃ saddaṃ sutvā tāvatiṃsā devā…pe… yāmā devā…pe… tusitā devā…pe… nimmānaratī devā…pe… paranimmitavasavattī devā…pe… brahmakāyikā devā saddamanussāvesuṃ –
Buddhist system does not work with ayatana (sense) but with citta (consciousness/mind). These brahmas exist with bhavanga citta, or unconscious state or sleep. When there is no other consciousness, bhavanga citta occurs. That process of bhavanga citta keeps going throughout samsara - from the start to now.
Remember the four paramattha: citta, cetasika, rupa, nibbana. There is no such thing as being at the fundamental state. Existence is the formation (sankhara) of citta and rupa. Based on this formation, cetasika may occur.
The brahmas with only rupa or only namma, as they are asleep or unconscious during their lifespans, they may not act or do. Also the various vipaka of their past kamma are halted and do not effect the brahmas who are the beings of sugati bhumi (pleasure realm). Only when they lose the life of brahma and are born in other realms, the past kamma will have effect again. Then they will again have ayatana (sense) and sense organs (that are in fact the reason and the states of suffering).