mid range tube amp for center channel.

Look at the cayin line. I believe the VAS II's, recently reviewed in TAS by Cordesman, could be the way to go.

Hi Liem,
What amps are driving your from L&R speakers? You obviously want the match with your center to be as close as possible. Generally speaking though the EL34 tube produces a fantastic midrange, I use a stereo EL34 amp in my two channel only system. There seem to be many low cost options out there, you may want to audition several, Sophia Electric comes to mind, but there are many other choices. Buying used you can really experiment, so have fun!