Thanks for the model numbers, for whatever reason, I was thinking you had a 4-bay antenna, not the 8-bay. In terms of noise performance, the PA-18 is better than any other consumer grade preamp.
If you're not willing to replace the CM4228, then one of the H-VHF 10 element Yagis could be combined via a UVSJ. The goal would be to get CH-9 & CH-11.
I would try a long UHF Yagi as a replacement for the CM4228 on the basis that panel antennas do not always get 'illuminated' equally.... ie. in many situations, some elements can be in 'hot' spots while one or more elements are in 'dead' zones. (Discussed here:
http://www.hdtvprimer.com/antennas/32bay.html) There has also been quite a bit of discussion and variety of opinion re. the fairly recent changes to the CM4228.
http://antennahacks.com/Comparisons/..._CM4228Old.htm
Adding a dissimilar UHF or combo antenna to the CM4228 is going to make the two 'fight' each other, which will give you less, not more. (Though combining an identical CM4228 might get you another 1 or 2 dB gain if done correctly.
http://www.hdtvprimer.com/antennas/16bay.html http://forum.tvfool.com/showthread.php?t=1024)
From scratch, my recipe would be the 91XG with it's own PA-18. An Antennacraft Y10713 with it's own PA-18. A UVSJ down stream of the power insertion blocks for the preamps. Then splitter and / or distribution amp as indicated by the number of TVs and length of runs. I would also be using a guyed push-up mast or better.
http://www.3starinc.com/telescoping_masts.html
http://www.antennacraft.net/Antennas/AntennasVHF.html
http://www.antennasdirect.com/store/...V_Antenna.html
http://www.solidsignal.com/pview.asp...Combiners&sku=
Finally, if you're blocked by trees, you've got quite a bit of challenge. KING is predicted to arrive at my location with an 'in the air NM' of over +70 dB... wind in the Douglas Fir limbs will still cause signal to breakup.