Aphex Aural Exciter.
Forget the suggestive sounding name! A BBC sound engineer with an interest in steam railways told me about this box of tricks from California. If you play records or tape recordings of any sort from many years ago you should seriously consider investing in one of these.
A big problem with the old tape recordings is that they have a very poor high frequency response. If you go down the conventional line of turning the treble control up and up all you will get is more and more hiss. What the Aphex Aural Exciter does (amongst other things) is to generate high frequencies that don't exist on the original tapes, it does this by generating harmonics of those that do exist. This gives a much better result than just turning up the treble control. It also does an excellent job on the low frequncies.
Have a look at their website: