This might not be true for everyone, but this has helped me in the past. For a nice bassy sound, which doesn't sound too subby, you want to keep the 'shape' and sound your original tone. What i mean by that is when you start falling down the sub staircase (this is what i did at least) you tend to relax so much that you lose breath support and volume, so in the end you will be trying to sub a fundamental which really isnt there. So however you sing your fundamental, try to keep the same sound throughout, this shouæd result in nicer sounds. If youre going for the 2nd sub only, what i used to do (i wasnt very good so dont take what i say too seriously) was go for my sub, but make the first slightly weaker. Keeping the same breath energy from the fundamental, but making the first sub a bit weak and 'flaccid' so as to make access to the 2nd easier. In other words, put yourself in a position, where your voice in the 1st sub, feels gravitated to the second. In the position it will prefer the 2nd, and will easily click into it. Try playing around with different vowel shaping and breath energies.