My quick opinions, and you might want to PM KJung, Vic, or TomBowlus, who are the collective authority on micros around here.
If you are replacing an SVT, I'll assume you place some value on dirty tone or at least a tube preamp. That narrows the field.
The Genz-Benz Shuttle 9 has a tube preamp that can give the tone a little warmth, but it's not designed for dirty tones. It's a punchy amp with a certain midrange focus that works pretty well in a crowded mix. I used one in this Skjold Design Bass review:
Will Greer Skjold Lions Pride Walkthrough.wmv - YouTube
The Genz-Benz Streamliner has a more aggressive six-stage tube preamp and a tube-driven EQ. It offers similar power amp options as the Shuttle series but carries the tone into much woolier, dirtier territory when pushed.
The GK MB Fusion has an all-tube preamp with three 12AX7s, somehow fit into a really small form factor - it's one of the smallest micros out there. It's capable of both punchy clean and dirty tones, and it has two switchable channels, so you can have both dirty and clean tones on tap. One of these is the newest member of my stable, and I'm still pretty enamored with how great it sounds and how small it is.
There are a lot of other great micros that don't have tube stages or preamps - the Markbass F1/F500, the GK MB500, and the Markbass LittleMark III among them. They all have something good to offer as long as you don't require the tube characteristic in your tone, although I think the LittleMark III sounds a little more tubey than the others. I was using one in this clip, tone run direct to the board via the DI. Hear it clearly beginning at 3:10:
Fina Dupa: Red Baron
Also consider the TC Electronics micros. I've never played any of them, so I don't have opinions, but I'm sure others will.
Enjoy.