you'll have problems just plugging a mike into a combo amp as the signal will be too weak. You'll need a suitable preamp unless you're using a DI box to give ourself monitoring and feeding the main signal into a PA.

Everything I've read about combo amps suggests Keyboard amps offer the most flexible usage for us cellos. Everyone has their own preferences. Note that yamaha make a dedicated combo for their Silent electric cello, and that Vector recommend a specific make Claris http://www.vectorinstruments.com/cellos/gear.html

I've been using the "California Blonde" by SWR for over a year. It's a bit heavy (~60) lbs., but is the best amp by far I've ever used. It has various inputs, including an XLR mic in which means you could use the direct mic approach. I currently use a pickup from England called
"The Band" which wraps around the cello and is removable after each session. I can either go direct or use a "BAGGS" preamp as a buffer to the amp, but as the amp was built with acoustic instruments in mind, If I don't need additional EQ(for feedback mitigation for instance) It's very clean as is. It's not cheap, probably ~$800.00 + something with a nice salesman, but it is capable of clean LOUD output and has lots of capabilities(extra speaker port, direct out, reverb, tuner out, footswitch, fx send and returns,etc., etc.) I've played with a very loud Jazz group with horns, peddle-steel guitar, and electric keyboard, and it totally Rocks!
Good luck, and post any other recommendations that sound better!
-- James Hoskins

P.S. I have heard that the guy who makes it lost some tool it took to make the pickup, and has discontinued them... hmm.

I described the rig I am using on another thread here but just read the comments about keyboard amps - I have played through Roland keyboard amps that sounded really good! Also they make an acoustic amp with preamp - very clean. Although when I reasearched this recently I found the Trace Elliot had more of the features I was looking for - their preamp set up is much better and offers more options for running to a sound board - I think the Rolands and SWR blonde amps sound better though.....