I`m sorry to have to say this but.. The reason there is no good 303 emu, which when I think of it really, I haven`t tried any in many years. But for the ten years before that, no good existed. And digital filters sounded strange.
THAT is because, my impression anyway, because software engineers come from school where very advanced filters are discussed. They fail to see the simplicity of it. And do not know the analog design, or general analog behavior.
There has been ALOT of nerding, when it comes to the 303. People talking about measuring the frequency response, doing weird stuff with the digital filter etc etc.
When I was a DSP newbie, I got quite close to a good 303 sound with the first filter I tried. Everyone else was talking about 60hz control rates, and filtertheorems, I ignored it all, ran samplerate modulation, and just took the simplest thing out there and put some clipping in the feedback path. It was a bit digital sounding because of the hard clip. Later I made a compressor set to limiting in the feedback path, before the clip, and that seemed to do the trick. The rez sounds a bit more musical, and less harsh that way. And some dynamics is actually often happening in analog components aswell. Ofcourse you won`t get exactly the 303 sound, but with some EQ`ing and similar approach to later sat/clip of the signal, for overdriven 303 sound, it is good enough for me, to not have any need for any real 303.
These nerds, some of them are on #musicdsp. If you ask them something, they will explain it in the most difficult way possible, just to seem more intelligent. But that attitude results in shit products.
Ofcourse back then 60hz was ok. Now I see things are a bit the other way around for some, looking at some posts on KvR, now they are going to do completely ridicolous things, and precalculate the filter, and talking about needing lots of oversampling for that. You just have to learn to recognize that and ignore it, and somehow try to find decent engineers, that can build products, without all that geeking around.
btw, I made this 303 track also, for fun, not any serious production, but just to fiddle around abit with this filter. http://www.paradoxuncreated.com/tmp/Acid.mp3
If that is good enough for you, it surely is for me, then 303 can be done well enough in software.