1985-1986 Model
Meaning: HX-S: (Unknown)
Bias: High (Type II)
Description/History:
  HX-S was TDK's possibly entry level cassette next to SA. The HX-S was also apparently more adaptable to CD than the SD or SA was. It also was the first Metal particle cassette on the market. HX-S has gone through a lot of change in many different forms, including today's CD Power in the US and CDing2 in UK and other countries. Although the HX-S is not made anymore, it is still a popular choice today.
More about the TDK CD Series
Compare to: TDK SA; Maxell UDS-II; Fuji DRII; That's CD/MH
BTG All-Time rank: 19

1996 Model
Meaning: SA-X: Super Avilyn Extra
Bias: High (Type II)
Description/History:
  SA-X is the more professional and more sophisticated brother of SA. Made with greater detail and quality, the SA-X is that kind of tape for those who required a better recording. Later on, late 80's and 90's, recording engineers utillized the SA-X for pro rmultitracking. It was still more affordable than TDK's own SM-X, and around the same price as SM. It was also a perferred choice for pro recordings along with the XLII-S from Maxell. The SA-X was discontinued in 2004 because of definite lack of sales and the ongoing push for mp#'s and Digital media. The SA-X still remains a very sturdy cassette even in its professional form as SM-X.
Compare to: Denon HD7; Denon HD8; Maxell XLII-S; Sony UX-S; Maxell Capsule UDXII
BTG All-Time rank: 12

1993 Model(?)
Meaning: SF: Super Fidelity
Bias: High (Type II)
Description/History:
  The SF was the SD's foreign brother, and had a similar dynamic range than SD. SF was great for lighter music. It can also record a vinyl record with ease, but you need to record in Dolby B or else you will pick up lots of crackles. You would need SA for good reproduction of LP's. SF can be your choice if you wanted an excellent quality cassette for jam sessions or a lecture in a large room.
  The SF had been a mainstay in Europe and Japan, and lasted many years. I am uncertain of its demise, but I believe it was somewhere around the mid-to-late 90's.
  I have only had the pleasure of trying a used SF, and I hope to someday find a new one from back in the day to fully test out its capabilities.
Compare to: Fuji DRII; TDK SD; Maxell UDII
BTG All-Time rank: tbd