To adjust the SNR, such as +5db, +10db

I used noise - this is a lot easier to do than using tone. Generate noise of whatever flavour you like and set the Intensity control to 3dB. Now use Normalize, and set the value to -67dB. You will now have a 3dB band of noise with the peak value at -67dB, and the noise floor will be at -70dB.

On the face of it, that will give you a signal to noise ratio of 70dB. But actually it doesn't, because the SNR is fixed by the number of digits you used to generate the signal. For instance, if you do the above as a undithered 16-bit file, you end up with a 96.32dB SNR signal with a band of noise in it at -67dB - not strictly the same thing at all.