Without getting too deep into ergonomics and workflow, often synth parts from the same synth (and sometimes different synths from the same company/designer) will make it easy to design sounds that blend well together. This is not only because they might share the same oscillators or filters, but also because the user interface encourages a similar approach to sound design.
On the flip side, synths with different user interfaces may encourage a different approach to sound design. Of course it’s not just the interface, other factors may include different oscillator and filter algorithms, as well as different envelope curves, keyboard/velocity scaling, portamento curves, etc.
If you don’t have a precise idea of the sound you want to design before you design it, you’ll find yourself more influenced by the affordances of the instrument. In other words, if you think “I think a snappy bass might work here”, you’ll go with what the instrument guides you to – the type of sound that the instrument makes easy to design, and the type of sound that sounds good quickly on that instrument. On the other hand, if you’re very clear about the exact sound you want (ie, you can hear it in your head) AND you know your instruments well enough to know how to get it, then you’ll “fight harder” to get what you want, but the end result will work better in a diverse mix.
If you’re working on a project with several different instruments and you’re finding a part isn’t quite blending with the rest, try this:
1) Pull the part’s volume right down to silence. Don’t use the mute button – actually pull down the channel fader.
2) Listen to your mix without the part, and IMAGINE the part. This is sound design, so don’t just imagine the notes or the type of sound (composition stuff – I’m assuming here you’ve already got that sorted). Really imagine how it sounds in the mix – frequency spectrum balance, dynamic range, depth, height (seriously!), interaction with other instruments, etc. This isn’t easy, and you’ll need to practise in order to get good at it.
3) SLOWLY raise the channel fader of the offending part. Stop as soon as it sounds wrong (or, you can hear the wrongness). Mentally compare the sound you’re hearing with the sound you’re expecting. Try to pinpoint exactly what is wrong with the sound, and what changes need to be made. Sometimes it’s just one aspect of the sound, often it’s a combination (which is why it’s difficult to get the sound to sit in the mix if you don’t know exactly what you’re aiming for).
4) Fix the sound. This is where it really pays to know your tools. Sometimes it’s adjusting the synth parameters. Sometimes it’s different eq, compression or other effects. If the sound is very wrong and you’ve used a lot of channel effects (such as eq and compression), remove them. Clear the channel and start again.
If you still can’t get it to work, you might need to go back to the composition. What are you trying to achieve with that part? Perhaps the rhythm isn’t working well against the other parts. Perhaps you need to transpose the part up or down by an octave (or less than an octave!). Maybe your imagination has failed you and the music actually needs a different type of sound, a different instrument.
Sometimes the music is simply better off without that part. Don’t try to shoe-horn in a sound just because you think it’s cool – every part in the music has to support the music. Ask yourself – what is the music trying to do here? How is this part supporting it? These are difficult questions to ask, and even more difficult to answer. With practice you’ll get better at it, and your music will thank you for it.