i've been trying to make a very smooth synth, adjusting the attack, velocity, etc. and can't seem to find a solution. sounds i'm getting are too harsh.
Even with the Heinseberg you can get smooth sounds. I'd start by feeding it long notes or chords and using the simplest waveforms. Then it really depends on what you're going for. A closed LP filter will give you a dark and moody sound. A fairly high HP filter will give you a very thin, ethereal sound. Then consider effects, they often give the character to the sound. A fully wet reverb can sound very good on a high frequency sound, very ghostly. A slight slow phaser can add a sense of unreality to it too. Without an example of what you want is hard to give more precise advice.
"Smooth synth" kind of speaks for itself in regards to the attack time...
just make the attack longer on your synth. That will reduce some of the harshness just by itself. Chances are, you probably want the release to be fairly long, too. I would describe something like this as being a pad. Usually these type of sounds come from waveforms with less harmonics, like sine waves (which have none) or triangle waves. Detuning the oscillators by a couple cents is a very good way to give depth to your sound. Additionally, you're going to want to mess around with your filter settings. Pads can actually be made out of just about any type of waveform, and the filter can be used to shave off some of the rough harmonics that come from sawtooth and square waves. Effects also help, especially reverb.
Which synth are you using? Have you tried Space? You can practically get silk out of it.