Actually, you will have a mix of VHF and UHF stations in Seattle to contend with...so you need reception of BOTH bands. Becasue of the severe multipath problems that plague Seattle, I suggest using VERY directional antenas: Channels 9, 11 and 13 will be on VHF, but the other stations will be on UHF. Most of your major Sea-tac stations are blocked by hills from your location, but, worse yet, FOX, KCPQ-Dt is in another direction, and will require either a separate antenna or a rotator to get reliably. Here's how I woud proceed:
Use a Winegard HD-7696P VHF-high-band+UHF antenna aimed at the group of transmitters that lie at 330 degrees, and use a Winegard YA-1713 VHF-high-band yagi aimed at KCPQ-DT at 290 degrees. Use a Channel Master Jointenna tuned specifically for channel 13 and connect the two antennas together, then run the output of the Jointenna into a Winegard HDP-269 preamplifier, then run the output of the preamplifier to the preamp power supply mounted indoors. If you want to split off the signal to other TV sets, then you can add your splitters downstream of the preamplifier power supply.: You should be able to power up to 4 TV sets from this antenna system. Be sure to separate the two antennas on the mast by at least 4' to avoid interaction between the antennas.
here are some links to help you!
http://www.channelmaster.comhttp://www.winegard.comhttp://www.pctinternational.com/channel ... lation.pdf