tmlfan said:
can someone explain why these are so valuable
In genera, metal tapes as a whole have a lower noise floor and can take higher signal levels without distorting than Chrome or Normal tapes. And The MA-R represents what is basically "THE" first real reference standard audio cassette. The metal frame makes the shell much heavier and thereby helps minimize vibration and stabilize tape path, and the quality of the actual magnetic tape is absolute top shelf. If' you'd ever heard one of these recorded on a top-quality deck, you'd instantly know what all the fuss is about.
IMHO, they are total overkill for boomboxes.
I have a handful of these and a few examples of most of the other super-metals (like the Maxell Vertex, Sony Super Metal Master, Denon MX-G and TDK"s later MA-XG) and have tested them on high end Nakamichi decks. And while I can comfortably say that you can hear the quality difference, the overall sonic benefit of using one of these MA-R's (or any other "super" metal for that matter) over a really good TDK SA-X or Maxell XLII-S is only marginal when compared to the tremendous difference in cost.
Have you ever heard of the "law of diminishing returns?" Where the cost for something goes up way out of proportion to the actual benefit.... that theory applies here perfectly, IMHO.
Still, aside of the Maxell Vertex, the TDK MA-R is probably one of the most sought after tapes (sealed) by collectors of audio tapes. You could say they are to cassettes, what the M90 is to boomboxes.