I don't know Platinum Notes but I seriously doubt that it can remove distortion. At least if the distortion is part of the audio track itself. If it is just distortion caused by using a too high normalization level, then yes, it will be able to do that, but Mp3Gain can also do that.
When you say that Mp3Gain resulted in distortion, then you chose a normalization level that was too high. The default value 89,0dB is usually a safe value where no distortion should occur.
Distortion can also occur if you max out song volume and playlist volume in Stage Traxx. If both are set to +6 dB and you play a track that also has maxed volume, it will most probably distort. Try to stay around 0dB for at least song or playlist volume.
Please note, ST2 does not read ReplayGain values. Only ST3 can do that. You can work around this by rendering the ReplayGain settings into the file. Mp3Gain can do that. And that operation is lossless and can be reverted.
What do I recommend? Stay with Mp3Gain, use 89dB as the normalization value, render changes into the file and import into Stage Traxx via iTunes Filesharing or Dropbox. Song volume together with playlist volume should not exceed +6dB in sum. If you use the global EQ, make sure to lower the gain if you boost frequencies.