Ok, 6 ohms isn't so bad. However, 85db is pretty paltry. Your AR should be able to drive almost anything sufficiently. Although, if you do the match your max output will be about 104db-105db at 100 watts which is not close to THX or rock concert levels. 400 wpc will get you to the 110db range. You need to borrow a pair of BIG monoblocks...
Question: Have you had your tubes checked? One leak/short/old tube could be adding to this if you are getting noise or harmonics when pushing it.
Question: Have you had your tubes checked? One leak/short/old tube could be adding to this if you are getting noise or harmonics when pushing it.