If you are having a rough time mixing external synths with plugins, try a new master EQ setting on your external gear. Master EQ provides control over where frequencies are boosted or negated. Many digital keyboards have this setting. When mixing in your DAW the hardest problem is getting the bass right. Many patches you create may not be EQ'ed properly to fit in your mix. If you have a drum track playing in your daw and the bass is muddy or fatiguing, try this on your outboard master EQ...
66hz-80hz -10 db or more on the Master EQ. This is a low shelf roll off.
This will allow space for your kick drum track (in your daw or elsewhere) to breath. Generally, you can leave this setting on with most patches and especially if you are playing a bassline on your external gear. The bass will no longer be muddy. Its subtle but the human ear doesn't need this information to deciper a bassline. If you are using analog gear you can press the EQ bass cut button on your mixer!
One reason to use this as your master EQ is to save time in case you forget to EQ bass out of a lot of patches. Then it can be switched on / off. There may be other Master EQ settings you want try, depending on what type of content you are producing. (For example, a cut in frequencies your vocalist normally sings).