This machines are revered among reel to reel enthusiasts and recording engineers. And for good reason: they sound great!
If you want to get the most out of these machines in my opinion it’s essential to replace the motor caps (Nagravox.com has them); this is actually a pretty easy job. Also important for sound quality is to have the playback and record PCB cards replaced with recapped ones (ebay seller apple-juices in Belgium have them at very reasonable prices) or have them recapped (see below) because the original ones use some pretty lousy sounding capacitors. Replacing the PCB boards is easy as they are computer style plug in types (very advanced for the time!)
The other thing I consider essential is to replace the pinch roller with a ball bearing type (Athan Corp’s or Nagravox.com) and to replace the right-hand fixed tape guide (IMO a big design flaw) with a ball bearing type (Nagravox.com); both jobs are easy to do. Both these things transform the sound of the machines: much more detail in the upper register, due to the increased tape speed stability. Everything on the soundstage just locks in.
A reliable and trustworthy service person is Jeff Knight at Technical Workshops in Melbourne. (email@example.com).