The two most common mistakes are bass and treble

Worst mistake for me is not to hear the speakers in your room with your gear before purchase.
I doubt impedance or sensitivity alone say a lot about the speaker sound. My worst speakers were with 8 ohm and north of 100 dB. Eversince I am around 4 ohm and below 90, increasingly satisfied with every next upgrade. And it is very true they require appropriate amplification. Every thing costs another thing. That’s life.

While I understand some of the arguments around high impedance and high efficiency speakers I also have to ask how often does this happen?  How often does a buyer mismatch his speakers and amps? 

Or is the argument if you aren't buying 16 Ohm/110 dB speakers you made a mistake?

The most important factor is not the necessary well match which is needed for sure between amplifier and speakers, and everyone learns that easily, but the most important factor is the room controls OVER the speaker and the room controls designed AROUND these specific speakers...A fact which is less easily understood and learned...

Acoustic is the KEY..... Not electronic design only and mainly, even if for sure it is important....In all audio thread all is about electronic design market....BUT it is secondary to acoustic... Nothing exceed the power of acoustic to give the most rewarding experience with ANYTHING well designed with an optimal S.Q./cost ratio....

And acoustic cost TIME not necessarily money.....And remember that acoustic is NOT only passive treatment but more than that, activation of the room...

Saying anything opposite will induce complete misunderstanding in beginners and worst will ignite upgrading chase of our own tail or worst, chasing of the moon....

Underestimating acoustic is the common error in most audio threads....I already paid the price for ignoring that myself....Nobody advised me CLEARLY and LOUDLY about this fundamental task....Then i speak to beginners here....Beware...

Dont think about a 10,000 bucks dac, speakers or amplifiers to be SOLUTION... Think about the time to create acoustical settings for your system.... It will cost time not so much money....

The 2 main factor about speakers/room are ASW and LEV not bass and highs....

An important factor in my enjoyment of music is sound pressure level.  I’ve seen that others here tend to listen at higher SPL than I do, and I find it hard to relax at higher than medium-to-low levels.  If I’m up doing things around the house, then louder is fine, but just kicked back enjoying the music, I like the music to be an element in the room and not consume the room.

When I first got into higher-level equipment a year ago, friends and family assumed that I would be rocking the neighborhood.  I think some believe I over-spent if all I was looking for was mellow.  But I can hear everything fine without the music bellowing at me.  

Like knighttodd, I have the ML 60XT’s but paired with the Parasound A21 amp. I also have two subwoofers, but sometimes I forget to turn them on and the sound is still pleasing.  They are efficient at 94, so not hard to drive even at 4 ohm impedance.  Everyone likes something different.  

Agree with the requirement to try them at home first. Many showrooms have odd acoustics and it’s not always possible to properly place the speakers in that room.