i used to own a pair of avant gardes for close to 3 years before selling them here on the 'gon. if i recall, the were spaced about 2 feet from the rear wall and about 9 feet apart. the quality of bass from them is really dependent upon the quality and type of amp used. when mated with an appropriate amplifier, the bass is definitely not boomy or bloated.
the tightest, quickest, and most natural bass extension i ever heard from them was when i used a pair of aragon palladium ii monoblocks. i ended up falling in love with tube amps and found that, while the bass extension was still deep, it seemed not as tight.
i replaced the gershman's with a pair of von schweikert dB99SE speakers. in my opinion, this is a real testament to the quality of the avant gardes. it took a $12k speaker to replace them.