Tough one. Your amp is not your bottleneck here.... major sonic upgrades will need some investment in getting your room layout and speaker placement better, ie. getting the mains off of the wall and spaced better. The stair wall is creating a bad acoustic reflection that is no doubt coloring the sound and narrowing your soundstage. Investment probably not worth it with your current room config.
But, if you are into buying an external power amp, check it's input sensitivity rating. This is the amount of voltage required to drive the amplifier to max output. Your preamp outputs are maxed at 1V according to the manual. If you buy an external amp with input sensitivity greater than 1V (to reach max output) you will never use all the power you just purchased. You will never drive the input signal on the amp side to 100%.
For examples only, you will never drive this amp much past half of its rated output spec, even with your receiver set to full volume as a preamp.http://shop.emotiva.com/collections/amplifiers/products/xpr5
This would be a better choice:http://shop.emotiva.com/collections/amplifiers/products/upa500