Rbf,
For rock/prog/metal music at louder, real life type volumes, clipping is quite common with many amp/speaker combos. It comes into play gradually as volume goes up. If it is noticeable with the Nova/OHM combo, there are two possible solutions. A more powerful/beefier amp for use with the OHMS or more efficient speakers for use with the Nova. My recommendation for the best end results for that kind of music assuming one likes what the OHMs can do in general at lower to moderate volume before clipping might become a factor would be to get the biggest, beefiest amp possible for the OHMs. Class D amps can do this effectively in a smaller lighter package than other types if needed. That is how I ended up where I am today to max out performance of my big OHM 5s. Smaller OHMS in smaller rooms will be less demanding of the amplification.