I too have found that using MIDI plugins and Mai Tai as well tend to max out my first two cores, but my other two cores barely even rev up to 15% when this happens. My computer is easily able to record 24 simultaneous channels of music and keep cores under 85% using Sonar X3, so it amazes me that Presonus can't manage get this issue under control.
Presonus pay attention please, and don't abandon your users like you did with the Audiobox 1818VSL !!
Focusing on getting your existing products running good before churning out new products would be a definite improvement over the way things have been going.