I agree. More doesn't mean better or even louder in this case.
Minimum impedance for that amp is 2 ohms which you already have with the two 10s. There is no way to add the two 8s without reducing the power the amp can output. Hooking the eights in series will do that and degrades bass quality.
Get better woofers not more woofers.
Or get another amp that can handle 1 ohm, or an additional amp just for the 8s.