[quote name='Prime_BASS' timestamp='1423133250' post='2681093']
You want one that doesn't colour the sound then. Some outboard pres are pretty much a colour box with an eq control, similar to the front end of an amp.
[/quote]
Not necessarily... Certainly you don't want one that colours it not to you taste but the whole point of the outboard is that you affecting the signal in some way, if only to clean it up, EQ it and boost it.
So the MXR, Sadowsky, Aguila, Bassbone are good candidates but I'd encourage you to consider those that add some dirt in your sound too, particularly if you like that vintage rock vibe (Geddy, Chris S, John Wetton). So consider also the Sans Amp range, EBS valve drive and connoisseurs' choice for bass distortion, the Dark Glass B7K.