Bridging NAD 3155 and 2155 to include a subwoofer

The only other way besides using a y adapter from the preamp,is using the speaker level connection.

You don't bridge anything. Use the 3155 to power the bookshelf speakers and your sub has a built in amp. Try the line level connection and the speaker cable connections on your sub and pick the one that sounds best. There's no reason to use the 2155.

atyourkitchendoor, I assume you know how to bridge the 3155 and 2155 to make them into 2x125W with a pair of Y adapters and connect to the super 8. With this configuration, your super 0s will be running at full range. 
the other option is using speaker level connection and high pass the super 0s.