In my opinion a pre-pro/amp combo is not needed. Especially one costing $1500-$2000 to drive M22's. I am still suspicious if the receiver you own is actually faulty since it only shuts down in 7.1 mode. I still think it is plausable that there could be a problem with your speaker wire leading to your newly installed surround speakers. I am not at all familiar with the HSU horn speakers though. If your receiver is indeed found to be defective and you have some money to spend you should consider a mid-upper end Denon receiver and if you have money left over after that spend it on another subwoofer over amplification.
