The ASL HEAD amps were popular in the early days of head-fi, early 2000’s. But interest in them dropped off like a ROCK as other manufacturers entered the headphone amp game (mid 2000s), and it was soon forgotten - like so many of the other early headphone amp makers. So if there was a last version - probably that IV - it would not have gotten into many hands, which explains why you can’t find info.
These amps were always considered a budget option, not very expensive - no more than ~ $300 - $400. Their OTL mode has high output impedance, so they would be paired with 300 ohm headphones like Sennheiser HD600. Its potential market today would be those seeking it for nostalgia, or those looking for an overtly warm / lush tube sound at the expense of some detail (much like the EARMAX of that time period). Honestly it could probably pleasantly surprise someone on that front. These kind of amps generate distortion like crazy but still sound pleasing as heck.