I have owned a 983 as my only horn for close to 10 years now. I got it when I was in college because it just "fit" me perfectly; it allows me to get closer to what's in my head than just about anything else I've played. Wouldn't trade it for anything.
If I had the money, would I get a contrabass to go with it? Sure, but it's certainly not a necessity.
@bloke: the y'all tube video doesn't have the best of sound, but the original soundtrack recording is pretty clear there. I really don't think he used a bass tuba for the Jawas solo; the sound's just too dark and thick.
Like many have said before me on this post, the EEb works fine as an only horn. I agree that for the big stuff (large orchestra and large orchestral works, etc.) a contra-bass is needed (CC or BBb) but I don't play in anything that would require something that large. I use the Willson 3400 EEb. It works well for everything I play in or would ever play. I don't agree with an earlier post that the Norwegian Star would work as the only EEb horn. I spent a couple of hours with one really putting it through the paces and in my opinion, it does not have the balz that the Willson has. When I really wanted to let loose, it couldn't handle it. I've heard and read that the MW 2141 is a good option as well as the Perrantucci EEb. I have not played any of those.
Now, with all that said, I also think the same thing could be accomplished with the Rudy Meinl 3/4 CC. Before all the EEb development, that was "the" all around tuba. So much so that I have considered on many occasions trading my Willson for a 3/4 Rudy (I prefer rotary valves - especially the older I get and the less I play). The one thing that has stopped me from taking the plunge is the sound and response on my Willson. It just sounds great! I'm sure age will finally make that decision for me.
I had a 2 month trial with both the Starlight and Norwegian Star and agree with Michael that I didn't think that either had the low down power that I needed/wanted/was used to. The 2040/5 that eventually chose did have this in spades, although I now understand through this forum that it's not a very popular choice.