I was just comparing the specifications in the manuals that are found on the Bryston site for the 6B and the 4B. This raised a couple questions:
1) The NOISE ratio quoted for the 6B (@ 29db / 23db gain) are slightly better than the 4B as follows:
6B: 112db @ 29dB gain & 115 @ 23 dB gain, whereas;
4B: 110db @ 29db gain & 113 @ 23 db gain.
How does the 6B actually outperform the 4B, when I would think their designs are identical and if anything, given space contraints, the 6B should be the amp that would face compromises? 
2) The 6B consumes 2100 watts for 300 wpc into 8 ohms (which is 17.5 Amps) - though the max. consumption for 4 ohms is not quoted it would be over 3500 watts and this would need over 30 Amps.
How would this ever be achievable, given that it appears it's designed for a standard 15A outlet?