Hi, just thought i'd chime in since i happen to be trying a few combinations using the sansamp, aphex bass xciter, and mxr. All 3 are pretty different, like corlando mentioned.
I'm not sure if using all 3 might be a good idea, as any of the 3 alone already has the potential to colour a basses' sound quite heavily. theoretically, you could try to get the tube sound from the sansamp, fatten it up with the AE and add a little crunch or something like tt from the mxr and use any of the 3 as a DI, but in practice i find it quite tricky with so many other variables from amp, to house speakers, to acoustics, to to..
Just talking about sound shaping alone, i struggle to put words to it, but the way the mxr shapes is quite unlike the sansamp. the mxr 'colour' thing doesn't really give the tube sound, but seems more like a preset mid scoop eq of sorts. the aphex, to me, 'fills out' the sound but doesnt compress it.
Currently, i almost always have the bass xciter around (though not always on the same settings) for the nice touch it adds to the sound, and i treat it a bit like a variable 'colour' button.
The mxr is a cool multi functional pedal, with DI, distortion, eq and colour, but I've recently used it less as the different components are taken cared of. A couple of soundmen have said that even a bit of distortion comes across as muddy on their house mixes (and thats only a little bit of it!).
DI wise, the aphex or other pedals already have an xlr out.
As for eq, between onboard, and on-amp, and sound board, etc, im sure we can find something passable without the mxr onboard eq too
Sansamps, a nice friend often allows me to play through a good ampeg with a xlr out to the board, so I only use the sansamp when that amp isnt around.