I stopped using all noise reduction when 3m 996 and ampex 499 came out.
In mastering, when I don't recieve analog mixes, I have many clients that want me to print their mixes to 1/2 inch, and master their projects off of the tape. Here's what I use: Otari MTR-10 with PLX extended range heads, and modified audio cards by Jim Williams. I have the machine setup to run ATR Magnetics and RMGI tape at +9/0vu when I record. Depending on the material I might hit the tape 3 db down from there. For some material I use 15 ips some i use 30 ips. With this setup, I know I'm getting at least 80 db signal to noise ratio if not better. Even on soft passages tape noise is barely audible, and this sounds so much better than using noise reduction. On a Studer A-80 using 1/4 inch ATR tape at 15 ips. You should be able to get really great sounding mixes that have very little noise.
I'm extremely particular about aligning my tape deck and you should be too. Find out how to get it spot on. Make sure you use a Hand D Mag
often, and always before before you align your machine. What sort of shape are your studer's electronics in? When was the last time it was recapped? Are your heads in good shape? I can tell you this. Years ago I used to mix to a Revox
PR-99 running 1/4 inch 3m 996 at +9/0vu 15 ips. Those mixes were not noisy at all, so you should be able to mixdown to and playback from an A-80 at 15 ips without using any noise reduction. The A-80 has such a gentle and quiet transport, its one of my favorites. Make sure that this transport is properly calibrated too.
I rarely ever recieve analog mixes these days where the mix engineer uses
noise reduction. I have dolby A and SR just in case. I have DBX too, but usually they're just taking up rack space.
If you go to the ATR magnetics website, you will find specs on their tape. These specs are referenced at 185nWb/m or true 0 vu. Even at that low of a level the S/N is really pretty good. This tape is made to run safely at a reference level of +10/0vu. One ought to beable to get really quiet referenced at +10. Some guys are running this tape at 185nWb/m at 30 ips. I see no point in this. I'd keep things in the digital realm if rather than hit the tape that lightly at that speed.
Anyway, my point is that reaching for noise reduction units would be my very last resort. I know you can get great sounding noise free mixes on an A-80 at 15 ips. if it is setup properly.
Good luck, and I hope this helps!