Hmm, I have this same setup. First thing to try is unplugging the laptop. I found my laptop was much less noisy when it was on batteries. But your readings still sound a bit high.
The Mobile Pre is known to have noise, but usually I get a baseline in the 40 dB area, not 65, which is definitely audible (and obviously not a true measurement).
It could also be a balance of your preamp gains and line-in setting. If the gain knob on the mic channel is too high, it could be introducing too much noise into the measurement.
For your test, spl calibration, and even the setting of the calibration file: try playing the tone with the generator and set the system/receiver volume to your reference or cal level (usually 0 to -10dB depending on your setup, tolerance for noise, room size, etc). Once that is done, try set the dial on the MobilePre until the green light lights up, but not the red clip. An intermittent clip of the red is fine, but make sure it isn't pegged red. Finally, set your line-in level to get a good reading (in the Calibration part of REW). It will tell you if the signal is too low.
I will warn you, you have to accept low signals (yellow band on the REW cal dialogue) or high in-room volumes (system volume up higher) to balance everything out.
If you do all this and Line-in is pegged and the signal is still low, raise system volume and see if that helps.
There is also a place to set the output volume of the test signals. Usually this is -12dB or so. You can raise this up to get a little more signal without changing the gain knob or line-in setting. I usually have to put this at -6dB or so.
Get this balanced, recalibrate, and see if that helps lower your noise floor. Good luck.