Option "3"--using the Infinity pair as rear surrounds--is your best option. Timbral matching of the three front channel speakers (the musical balance should, ideally, be identical in the three front channels) is essential in home theater systems, so the sonic character of dialogue voices and instruments doesn't vary across the front soundstage. Buying three front-channel speakers is an excellent place to begin. And while the timbral match of the surround speakers to the front speakers is nevertheless important, you can use the Infinitys as surrounds for the time being. After a time, you may find the timbral mismatch of the Infinitys quite annoying and want to upgrade to surrounds that match your main and center-channel speakers.
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