Hi,
Your receiver has plenty of power for a smaller room and if they're on a desk near you, even more room. If you're clipping them and turning it up so loud that it's chugging along, maybe its a big room or maybe something else is going on that you're turning it up really loud and hitting some limits? Unsure without more info.
If you want more mids or mid-bass or upper bass, it's not really the speaker so much as it is placement and the room itself relative to your listening position. Even a cheap speaker will have a fairly good response for mids and upper bass.
Cabinet volume greatly effects efficiency, so if you want more output, a bigger cabinet is a quick way towards that.
Very best,