I have just tried this on my FreeBSD-box. energyXT standalone doesn't make sense as you don't have an ASIO driver for BSD, but within a VST host (such as Audiomulch), energyXT works like a charm. It even starts quicker and doesn't seem to be much slower than on Windows (I'll test CPU consumption later).
Screenshot of Audiomulch+energyXT+asynth on FreeBSD:

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