I had not used Audacity in quite a while and had no idea that it had been improved so much as to include a sample noise remover filter. This is most excellent for a freeware offering. Although not as elegant and easy to use as Amadeus, if you are just doing a few tapes here and there, this would be the way to go to keep expenses down.
Forget overdubbing. You just need to get one good original from your tapes. How you do that depends on the equipment available. The Technics deck would be fine, IF it plays the tapes well. Make sure the heads, rollers and capstans are clean. Plug the output RCAs into the Mac and record the music accordingly. Make sure to save this as your master recording and burn the files to CD. Never do any noise reduction on your original!
Once you have a duplicate copy, start the noise reduction process with Amadeus or Audacity to remove tape hiss. If you want more bass or need to bring out some highs, use the EQ filters, but do not OVER EQ. Use your Normalize filter to make sure the audio level is appropriate.
This should result in a good archive of precious tapes.