...BTW, are you 101% sure the stuff you want to transfer has not made the digital world at all? You don't have an idea how much music Japan put out on CD in small number....
A great point which I immediately thought of when I read Adam's (Fred's) post. For years, I've been amazed at how many rare albums have been released on CD. Many have sold far more, albeit still limited quantities, on CD than were ever sold on LP/prerecorded tape.
Regarding noise reduction, a quick primer on how they work might be useful.
1- dbx II is a noise ELIMINATION system, not noise reduction. Tapes recorded with it REQUIRE it in playback. Likewise, playback with dbx on tapes NOT recorded with it will simply screw things up. This system compresses the sound in a 2:1 ratio in recording, and expands on a 1:2 ratio in playback. To my knowledge, only a handful of prerecorded cassettes have ever been made with this system.
2- Dolby B ONLY works on high frequencies. By boosting them up by 10 db in recording, then cutting the highs by 10 db in playback, noise is reduced. So, why play back a Dolby B encoded tape without Dolby B? Because you can compensate for the lack of high end of the physical tape stock, as well as part of the high end deficiencies of the playback cassette deck. The accuracy of the high frequency information is compromised in this process. Yet, I've done exactly that when playing back on cassette equipment with a limited high end. Accuracy may be compromised, but the results are often very pleasing.
3- Dolby C was used on some prerecorded tapes. In many ways, it is a more sophisticated version of Dolby B. It works on a wider range of frequencies. s/n ratio is as much as 20 db. Yet, I would ONLY decode with Dolby C IF the tape was recorded with it.
Also, be sure to use the correct settings on the playback deck for tape formulation. Just as noise reduction playback settings can effect the tonal balance, so can playing back with the wrong setting. Most prerecorded tapes were recorded on normal iron oxide tapes. However, many 80's prerecorded tapes were done on Chromium Dioxide tape, or more commonly, cobalt doped iron oxide tape formulations (similar to TDK SA or Maxell UDXLII), which mimic actual "chrome' with lower head wear and a better low end.
Finally, I'll reiterate winopener's comments. As much as anyone, I understand how rare recordings can have sentimental value, as well as a fondness for the music itself. Still, CD releases of these may add greatly to the cost of the project. Still, those CD's will likely sound better and save a whole lot of work.
Good luck!