I have already covered several videos about the bassline, but this topic I missed to explain so far.
When it comes to the bassline, my advice is always to make your bassline-flat, using only a root-key note for designing your bassline. If you want to make a melodic bassline and change the notes while the bassline is going, I would still advise that you keep it flat until you design it properly on the root-key. Listening to one note only for several days may help you go deeper and make it close to perfect, as when you change the notes, you won't have this hypnotized moment. You will not be able to clear all frequencies and have an overall picture of the notes' dynamic (transient/body/tale) as you would be while listening to only one note all the way. After you design your bassline on root-key, you can start making bassline melody and solve problems that may happen when you change the notes. Usually, 2 things can go wrong.
The first thing is that you want to make your synthesis properly from the beginning. Start-going to the waveform is really important, and you should be sure that the start point will always be the same, and that waveform will start from the same position on each note. This is a common problem while using Sylenth for bassline synthesis, as Sylenth sounds the best when the phase knob on the oscillator is set around 160 degrees. When the phase is set between 0 and 180, there will be changes in the bassline's sound as when we pitch/transpose notes, the waveform length is changing (pitch), and that's why this point of 160 degrees on each note is different. Sylenth is really nice sounding VSTi instrument, but I wouldn't recommend it for melodic basslines, a much better and easier option for melodic basslines is Serum. You can get even better results by using the Serum.
The second thing that might go wrong in this process is that because of the big bassline processing and many multiband processings, it might happen that when the note is changed that those notes can become louder because of the processing. You can apply several processes for solving this problem, and it is a real challenge to write all of it down, so I will recommend seeing the video below for all the details regarding this issue.
Here is XML file for Cubase Users: DOWNLOAD