What on earth is Loading and Gain

I feel like an idiot, but I have a Conrad Johson EF1 which has 47kohm, 1kohm (I think), 500ohm, and 200ohm. I have transfiguration temper and also a shelter 501.

I've decided to use the 501 for a bit. This has a 4mv output, the temper is 0.25.

I found my sytem sort of 'clipped' with the old settings of 52db gain and 200ohm.

I moved to 47kohm and this 'clipping' stopped, but the sound is now quite shut in, quieter, deeper, and darker.

Why is this?

can somebody explain this all to me?