I'd say that your choice of components is near optimum for your location. There are no UHF stations strong enough to overload the Winegard 4800 amplifier. Yet the two super strong VHF stations could overload that amp it if the VHF/UHF filtering in the 4800 is inadequate. KYFQ FM 91.7 is also extremely strong at roughly -22 dbm at your location. Again the filtering of the amp should minimize that, as should the smart way that you've combined the VHF antenna after the UHF preamp.
To rule out a problem from VHF or FM, try just the 91XG and the preamp and see if KIRO comes back. If so, you are chasing down interference which is likely caused by overload. If not, trees may have grown in front of your antenna. You can search for a location that has fewer trees.
Another potential solution for you is changing the antenna height. Your signals from KIRO likely refract from Green MT, which is 3 miles from you. The angle up to Green Mt is 4.3 degrees. I use
www.heywhatsthat.com to calculate elevation angles. Some have found that tilting a 91XG helps. I prefer to find an optimum height where the ground reflections cause the signal to elevate upwards. In your case the math says that optimum heights for channel 39 are 5', 15' and perhaps 30', but that's for flat ground in front of the antenna.