"Hard or Easy to drive" is an industry term meaning a level of "efficiency", not how loud you want to push your speaker system. A speaker which is hard to drive is less efficient than a speaker that is easy to drive. Often, but not always, high end speakers are easier to drive than low end speakers and smaller speakers are easier to drive than towers. I have seen several exceptions to the above rule, but its a fair assessment for our discussion here. Being hard to drive does not make a speaker "bad" just as being "easy" to drive does not make a speaker good. It is simply one of the many personality characteristics that one looks at when pairing a speaker/receiver together. It is also one of the most important because with an inefficient speaker set paired to an under powered receiver, then this can spell difficulty even for the casual listener. It makes speaker placement, wiring, and even speaker isolation much more necessary to achieve clean sound.
My wife can hear a bad combination even at nominal sound levels, and it downright gives her a headache at reference levels. I am not as hard to please, but I can also pick out the difference quite easily, as most audio aficionados probably can. Good combinations can make music come alive, while bad combinations can bury it. Again this is regardless of sound level.
I will use an imperfect car analogy. Take 3 cars- honda civic, Porche 911, and Hummer. Each has a different efficiency, but all run wonderfully. The efficiency in this case has much to do with the body weight and size of vehicle. When you put the honda engine in a Hummer, even driving 5 miles per hour, the honda engine is going to have noticeable deficits, while the stronger Porche engine will have less deficits and may actually do well in the Hummer body. You would possibly blow the honda engine if trying to force it to high speed in the Hummer body.
The same is true with an under powered receiver and hard to drive speakers. Your sound simply will not be as good, regardless of sound level. And the higher you push the sound level the more noticeable the deficits will become. The bass will become muddy, the mids will bleed into the highs, and the highs will become ear piercing at reference levels. Not to mention the fact that if you ever do want to push them to reference levels you are going to take a serious chance of overheating the receivers built in amplifier system, causing it to blow and most likely also ruining your speakers at the same time.
As a side note: I was not trying to muddy your decision by producing a bunch of stuff you probably don't care about and most likely won't notice in your room at the sound levels you are considering. I was just trying to give some insight into why I suggested you not choose the MB's, which are actually the ones most would suggest you get.