It would be best to have an amp with 2 ohm taps on the transformer. Or at least a Push-Pull amp with enough feedback for a decently low output impedance. According to the published charts, the Cremonas have 2.3 ohm dips at the bass frequencies and also large phase swings at the same area. This is the frequency range where the most power delivery is required, and is most demanding on the amp. Having to dump large amounts of power into the speakers to satisfy this demand, while reducing the damping factor significantly(due to the low impedance dip at this frequency)is not an ideal situation. I'd recommend an amp with an output impedance of lower than 0.5 ohms for this application. This will provide you with a damping factor of at least 4 at these difficult points, and help to ensure stability of the amp and control of the drivers.
IMHO.
