ryclarck, Bob,
thanks for your answers. I've actually done it with Noise Reduction (I'm experienced enough to know that Hiss reduction usually performs bad), in 2 passes, but removing 100% Noise each time. I suppose, 10-15% Noise reduciton each time would require me to do no less than 8-10 passes, since there is really much noise. I also didn't grab a new noise sample each time since I was not using a Noise sample from the recording but the section of the cassette where there was no recording at all (https://dl.dropboxusercontent.com/u/17034600/Tape%20hiss.wav- if I grabbed a sample from inside the recording, I would lose the concert hall "atmosphere" along with the actual hiss).
Are 8-10 passes of low-level (15%) Noise reduction really worth trying? Also, how should the "Spectral Decay Rate" setting be set? I've noticed that lower values of this setting than the default one (say, 10-20%) allow me to get somewhat better results.