I had the Infinity Kappa's in my last truck (MB in my current truck after not being happy with the Polk db's), and a 12 behind the seat. The Kappa's were a bit on the bright side, I ran the crossovers at -2db and typically the treble at -1 to -3 most of the time.
I like Infinity's in boats, bright is good in a boat in general. Never heard the Diamonds.
Keep in mind those Infinity's are 2 ohm so your wiring configuration may be different.