Well, it's actually normal for a compressor. The low frequencies hold much more energy and will trigger the compressor much earlier compared to the rest of the spectrum. Soon sidechaining will be implemented. Then you will be able to filter out the low frequencies on the sidechain channel, so they don't trigger the compressor.
If you don't want to wait, the signal to noise 168 compressor "big" preset, which has a build in side-chain high pass, is available.
You could also split the audio into different bands and compress each separately (mutliband compression)
As far as I know, Nebula Reverb and Nebula aren't intrinsically different. One can be changed into the other just by editing and saving parameters on the Master page (load time, buffer size etc.) The two versions are included as such solely for convenience's sake.