Browsing through Wiktionary, I ran across a note in a-stem declension tables (like žena) which claims that -asъ is the expected Balto-Slavic form of locative plural, which is however found only in some Old Czech documents and is replaced elsewhere by -axъ in analogy with the other declension types. I've never seen this claim in literature, Czech or any other, and it does not seem to be substantiated on Wiktionary, so I was wondering if anybody here might have some pointers on this.
It seems the use of x in -axъ is thought to be related to the "ruki" law. However, becasue this is not the usual context for the application of this sound law, it is supposed that some kind of analogy caused the original *s sound to develop to [x] in this ending. As Atamiri's answer mentions, some evidence that the locative plural originally had the consonant *s comes from Baltic languages like Lithuanian.
According to "On The Origins Of x in Slavic", by Ondřej Šefčík, in Linguistica Brunensia 61, 2013:
We can formulate Pedersen’s Law for Common Slavic as: “If Indo-European *s is preceded by *ı̄̆, ū̆, r, k and is not followed by an obstruent, it changes to *š, if followed by a palatal/front phoneme, or to *x, if followed by a non-palatal/back phoneme.”
The collateral effect of Pedersen’s Law and the source of a secondary analogical x is the x of the Slavic sigmatic aorist when, besides regular forms like OCS rěxъ, prosixъ, krychъ, there arose by analogy such new forms as znaxъ and nesoxъ. The original forms that do not show the effect of Pedersen’s Law are attested in OCS bljusъ, grěsъ. Similarly, the locative plural OCS ženachъ is due to analogy with the regular synъxъ.
Sorry, I don't know anything about possible attested examples of -asъ, but I hope this explains why it might be considered the "expected" form.