Signless is not a jhana and is not neither perception nor non perception. For example, both <a href="https://suttacentral.net/sn40.9/en/sujato">SN 40.9</a> & <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> appear to say the signless can be attained after the sphere of  neither perception nor non perception.

However, other suttas, such as <a href="https://suttacentral.net/sn41.6/en/sujato#8.2">SN 41.6</a>, repeated in MN 44, say the signless can be attained after emerging from the cessation of perception & feeling.

Therefore, the impression is the foremost signlessness does not occur directly from a furtherance of "tranquilization" (as described in <a href="https://suttacentral.net/sn36.11/en/bodhi">SN 36.11</a>, MN 111, MN 26, etc) after the sphere of  neither perception nor non perception. Instead, it appears the signless occurs after emerging from  the sphere of  neither perception nor non perception; or otherwise, emerging from the cessation of perception & feeling; when the mind becomes increasing conscious rather than less conscious. 

In conclusion, MN 43, AN 6.13, AN 7.56 and particularly <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> & <a href="https://suttacentral.net/an6.60/en/sujato#8.1">AN 6.60</a> appears to clearly say the signless is not the foremost in liberation; is not emptiness (*sunnata*); and is not Nibbana. <a href="https://suttacentral.net/mn121/en/sujato#11.1">MN 121</a> says: 

> ‘Even this signless immersion of the heart is produced by choices and
> intentions.’
> 
> ‘ayampi kho animitto cetosamādhi abhisaṅkhato abhisañcetayito’.
> 
> They understand: ‘But whatever is produced by choices and intentions
> is impermanent and liable to cessation.’
> 
> ‘Yaṃ kho pana kiñci abhisaṅkhataṃ abhisañcetayitaṃ tadaniccaṃ
> nirodhadhamman’ti pajānāti.

<a href="https://suttacentral.net/an6.60/en/sujato#8.1">AN 6.60</a> says:

> Take the case of another person who, not focusing on any signs, enters
> and remains in the signless immersion of the heart.
> 
> Idha, panāvuso, ekacco puggalo sabbanimittānaṃ amanasikārā animittaṃ
> cetosamādhiṃ upasampajja viharati.
> 
> Thinking, ‘I get the signless immersion of the heart!’ they mix
> closely with monks, nuns, laymen, and laywomen; with rulers and their
> ministers, and with teachers of other paths and their followers.
> 
> So ‘lābhimhi animittassa cetosamādhissā’ti saṃsaṭṭho viharati
> bhikkhūhi bhikkhunīhi upāsakehi upāsikāhi raññā rājamahāmattehi
> titthiyehi titthiyasāvakehi.
> 
> As they mix closely, they become intimate and loose, spending time
> chatting, and so lust infects their mind.
> 
> Tassa saṃsaṭṭhassa vissatthassa pākatassa bhassamanuyuttassa viharato
> rāgo cittaṃ anuddhaṃseti.
> 
> They reject the training and return to a lesser life.
> 
> So rāgānuddhaṃsitena cittena sikkhaṃ paccakkhāya hīnāyāvattati.

Therefore, when the Mahayana attempts to say the signless is superior to Nibbana; that Nibbana is merely a sign or concept; that Nibbana is the same as Samsara; obviously the Mahayana are wrong.