stevenl -
and the reason your cable modem just like Khand said is split off is to isolate your line, although the amp will increase the fwd signals it doesnt do the same for the rev. so a 8port amp has 10 DB of loss on the return a 2 way splitter only has 3.5 so putting it on the splitter saves your return path. (upstream)
The 8 port amps we use PCT brand are 4.5, the previous ones we used to use were 4.0. What brand of 8 port amp are you using with a 10db? Our 4 ports only do 8db. Our 1port uses 15db. Now, we switched brands about 4 months ago, but even our old ones were close (within .5 to 1db) of the new.