Alnico and HT? I don't equate/think of the warmer sound of Alnico with HT.
5 won't be cheap either. I love my Fostex F200A (8 inch) Alnico drivers that I use in my audio system, and they also offer a F120A (5 inch) little brothers that could be used for rear channels. The 5 drivers would run about $2,000.00 (without the EnABL treatments that I'd highly recommend), beam high frequencies, and have somewhat limited sound pressure levels for HT. But the F200A is rated F3 = 30 Hz, so need for a sub would be minimal.