From an explanation I got a long time ago (can't remember who) the #6 and #7's are the same. Not sure if there is any truth to that or not, but maybe someone with a little more knowledge can chime in. I've had good luck with both Fenner and Rockford...but have much more experience with Rockfords and never really had any problem with them. I will say I love the sound of Marzocchis and it's undeniable that they build more pressure. Maybe one of the Black Magic guys can chime in since they sell all three options.