The pops are a widespread issue. I also have it on power up and power down on my Hypex/Ghent amp. However, it is not that loud at all, so no real concern there for me.
The volume adjustments are a function of the preamp (e30 in your case). I tried a DX7PRO as a preamp, and also noticed you have to go pretty high on the volume scale to get into the power. I no longer use this as a preamp, though.
When you say a "converter" do you mean rca to xlr converter box? This may be fine, but usually cables designed for this purpose are recommended (easily available on amazon, etc), with xlr on one end and rca on the other. For shorter runs in home use, rca should be fine if you have no ground loop issues. My understanding is you will still be using the full power of the amp, but feeding it 2v instead of 4v requires more amp gain for a certain volume, so yes it can affect the max SPL, all else being equal.