While I was playing with the crossover point on my Yamaha, I could really cross the subs at 80,90,100 without much difference. When I went 60 on down it really shut down the midbass and tone quality of the system. It became very thin. I don't know much about how it all works...I just plug in the numbers until I think it sounds good.
I do think the plugs were a plus for my situation. It takes all the guilt out of having the Sierra's in the cabinet.
Sometimes I wish the Rythmik subs had been an option earlier...I might have ponied up for a pair. As it is the ULW10's only cost me $280 each and they perform like champions. They are not limitless but when played within their design limits they sing and really extend the Sierra's. Having the dual subs located with the L/R Sierra's makes everything so seemless that it feels like I've got giant floorstanding speakers.