Tri-way impedence w/3065's

Systems

I would like to add a third speaker, an 8" woofer, to aide in preventing me from overpowering my 3065's as well as adding slightly more bass to the system. The amp being used has only 2 channels which currently powers my 3065's. So, I plan on doing this by adding this speaker across the + of my amps left channel and the - of the right channel since it is only a 2 channel amp. I plan to use bass blockers to block the lows, around 80Hz, from going to the 3065's. And use a low pass only in-line filter, @ 100Hz or so for the 8" woofer. My question is: If the speaker I would like to use has an impedence of 4 ohms, what will be the the impedence be on each channel since there already exists the impedence of the 3065's.

Systems

One more thing I would like some advise on. Would I get better sounding bass by using two 4 ohm speakers wired together in series between the left and right channels of my amplifier vs. one 4 ohm or one 8 ohm speaker wired between the amplifiers left and right channels. Reason I mention one 4 ohm and one 8 ohm speaker is I do not yet know what the impedence will be using either one of these choices in a tri-way setup.