I was interested in this issue a while back and scoured the net in search of guidance. As you might guess, the most common conclusion is that if you want the sound of an upright bass get an upright (and a roof rack for your car--there is a reason that Leo Fender invented the electric!).
You would think that Behringer or some cheeseball manufacturer would invent and sell a magic pedal that you could use to produce the sounds of a vinatge Kay upright. But, alas, is doesn't seem to exist unless Line 6 is hiding something in their Bass Pod!
You could try rolling off some treble at 2k. I read somewhere that this helps approximate an upright tone. And, if you really, want to get aggressive, you could try some fretless electric basses. You might also want to look at different types of strings (nylon wrapped). There is also the upright electric bass and the ABG (acoustic bass guitar), but I'm not sure either would be worth the trouble. Electric uprights are expensive (even the Goldtone is about $1299) and they still leave something lacking in the tone area according to what I've read. The ABG is fun for unplugged stuff and you could certainly record with it, but they typically lack volume and really don't sound the same as an upright. Most are oversized guitars, although the Tacoma Thunderchief is clearly more beefy than, say, the Martin. Unless you are starting a Violent Femmes tribute band, it's hard to recommend.
