Alright, I've done a little experiment see see what the volume levels are like. I used the Measures App, which may not be too accurate overall but should give a good idea relative to one another. I used the range of peak values that I saw, which are momentary maximums (not the overall maximum or average, as these don't give any relevant information in this test). All of these were done on a hard desk with no padding, and all were bottoming out the keys.
IBM Model M (Buckling): 63-69
Blackwidow (MX Blue): 66-72
KPT-102 (Simple Alps-Blue): 67-72
QFR (MX Red): 65-72
Das Keyboard (MX Brown): 65-71
As far as the decibel levels go, they are all actually pretty similar. Now, that does not take into account the perception of each sound (how irritating it can be), nor the wall penetration of each sound. It really is just to give an idea of the average peak levels when typing. The overall average could be quite different between two keyboards that have the same average peak levels, since the amount of noise between peaks varies dramatically.
It would also make a different to have these on a rubber mat, not so much with peak dB levels, but especially with wall penetration. The Model M did best in this aspect as it seems to have a soft bottom-out that doesn't cause a drum effect on the table top without using a mat. Another option is to put the keyboard on your lap when you type, which can dramatically reduce the overall noise. This especially helped the MX Browns and Reds and brought them down to what could be considered quiet levels; the quietest of this test.
Avoiding bottoming-out can also reduce noise levels dramatically on MX switches (especially Brown, Red, and presumably Black), although it would have little effect on MX Blues and no effect on buckling springs.
Overall, they are all pretty loud if you are using them in a quiet environment, such as a house with people sleeping. If you really want to be able to do some serious typing at night, I'd recommend a non-clicky keyboard, typing on a soft rubber mat, or better yet, your lap.
I also tested my Apple aluminum BT keyboard. It can in at 45-50 dB on a soft surface. My office with me still is around 37 dB. Consider that sound is exponential, so that Apple keyboard is dead silent outside of the room, and extremely quiet inside it.
All of this to say... Get whichever you would like, and just use it in the right situations. It's very common around GH to have several keyboards and to rotate them through, so you would likely still be able to enjoy them all, just at the appropriate times.
PS. You're welcome to disagree with my findings. I've tried to be as objective as possible, but I entirely concede that the number of factors are too overwhelming for any seriously accurate conclusions about which keyboards are the loudest/quietest.
