You can still do it offline if you are savvy with wav editing. I just paste a custom tone at the front of the tones generated by NAT. The 'custom' tone is for synching everything later, so I make sure its suited for that purpose. Then, I just play the whole thing in a loop, and record what comes out of my 'hardware' into a new file. After maybe 4 passes, I save the result. Then it helps to have a multi-track editor with a sample accurate view, so you can line up all of the 'takes' in a stack. With 4 copies, you would set each to -12db, then mix down into a new copy, with maybe around 10 to 12db noise reduction. It depends on the nature of the noise. To get another 10-12db you would need 16 copies and for another 10-12db you'd need whatever 16x16 is. Somewhere around a million I think, so I draw the line at about 16 copies. Of course after you mix everything down to the final file, you'd need to remember to remove the synch tone at the file start.
This might all be obvious to everyone, but since I'm dealing with this issue right now, I thought I'd share.
Oh and if 16 copies sounds crazy, it kind of is, but I am dealing with some fairly noisy equipment. Also, the reason I posted about this is because I noticed an issue with the repeat function with online sampling. It doesn't seem to work, at least not how I expect it to. If I make a session with only 1 dynamic sample level for a test, and 5 repeats, it does repeat 5 times, but the resulting .wav it creates is definitely NOT an average of those 5 repeats. After I noticed this I even did an experiment to verify where I dropped the fader of my soundcard's output about halfway through the first repeat, then did the opposite for the 2nd (with only 2 repeats total). The resulting .wav file created by NAT only had the first half of the tone sweep. So its like it kept the first pass and just threw away the 2nd. What's up with that? I assumed that the wav file would be the result of all of the repeats being averaged together, is that not how it works? Anyone know about this?