Well, with the 7125 you'll have seven channels of amplification and could use two channels to drive the Boston Acoustics. Those two channels could be connected to the 990's second zone, which would allow you to pipe one source to the main system and a second source (either the same one as in the main system, or a different source entirely) to the dining room.

The problem with the Kenwood is probably the result of overheating due to driving a 4 ohm loads - receivers rarely are well suited to dealing with a 4 ohm load. The 7125 is rated at 125W at 8 ohms and 190 watts at 4 ohms - it'll have no problem with a 4 ohm load.