[Philbo King]

There are a couple tricks that make Subtract mode in ReaFIR more versatile.
1. As somebody mentioned, always put ReaFIR first in the effects chain.
2. Once you capture the noise spectrum, use Ctrl-drag on the graph to vary the amount of reduction. Lower = less reduction. Lower it till you no longer hear those 'bubbly' artifacts, then go a few dB further.
3. You can use parameter modulation in ReaFIR to back off the moise reduction during loud passages. Touch the wet/dry knob, click the Param button, select audio source, click the 'negative' tick box (so louder audio turns the wet/dry knob towards dry), and adjust the strength to taste. The idea is that when the audio is loud it buries the tape hiss, so you don't need to remove the noise until it goes quiet.

If the sound was captured by a cell phone or other portable, it's probably heavily compressed. You might try a downward expander to restore a bit of dynamic range. I use Cockos General Dynamics plug for this. Downward expansion has a side benefit of reducing the noise floor further as well.