It sounded ok? I guess.. seemed to be just as loud as 6550's but the bottom was "lifting" up differently when muting etc. I like the 6550's better in this amp. The factory setting on the 6550's was in the 40's does this seem too underbiased and stressful to the power transformer? When backed off to around 38 it sounded weaker with less high end cut... didn't make any sense to me, it should be brighter at that setting.