I will agree with everyone else--the Center speaker is the MOST important in a HT setup. Most of the dialog, main sound effects and even the indicental music on movie soundtracks gets routed through the Center speaker.
The Front R/L speakers are for front-side ambient cues, and to give the soundtage some depth and width. Rears are for soft ambience and special effects. The choice of Center speaker should be given the same care you'd give to choosing your TV.
Matching the voice of your Center to the mains is also critical. If it's not matched well, you will hear all sorts of weird interaction, frequency humps and dips, and it can even effect imaging on movie soundtracks.
If you have an SACD player, or are listening to music in a multi-channel mode, matching the voicing of your center to your mains is even more critical.
So if you can't afford a brand new ML ESL center, I would suggest either getting a Fresco, or trying to find a used Logos in good condition. The Logos is, to some of our ears, the most musical center ML ever made--it hs full bass, delightful mids, clear highs, and a wider dispersion pattern than the newer "inverse curve" panels of the newer ML Centers.
And besides, you will be VERY hard-pressed to find a center speaker from another manufacturer that voices similarly to ML main speakers. Perhaps the Sunfire speakers (with the ribbon drivers) might be a good match, but I think that trying to match a "traditional" Center speaker to a pair of Vistas or Vantages will be fraught with heartache...
--Richard
