By the way these are the speakers I'm interested in and I'm actually kind of probably going to buy a couple of the ones I'm mentioning.
MA Bronze 100
Klipsch 600II
WD EVO 4.1
Polk Reserve 200
Elac Debut Ref DBR62
And now, to your list of speakers:
Monitor Audio Bronze 100
SoundStage has measured them. And so has
AudioScienceReviews. Both indicate that these aren't bad speakers at all. Put them high on your list. Make an effort to hear them.
Be sure to look at the Impedance and Impedance Phase curves. And as an example of exaggerated sensitivity, Monitor Audio says these speakers have a sensitivity of 87 dB, but SoundStage says they measured 84.4 dB (averaged over the audio range of 300Hz-3kHz, on Listening Window, measured at 2.83V/1m). 84.4 dB is believable, but not 87 dB.
Klipsch 600II
Klipsch speakers are not for everyone. Many, including myself, think they are very bright sounding, to the point of sounding tinny or harsh. Others like them. So be sure which type of listener you are.
Also, be aware that Klipsch has historically been known to highly exaggerate it's sensitivity numbers. For the RP 600 II, Klipsch claims 94.5 dB, and Erin measured 86 dB. Personally, I have great trouble believing any sensitivity claim for a home speaker that's more than 90 dB.
Erin’s Audio Corner says these “are among the more neutral sounding Klipsch speakers I’ve listened to and the data shows reasonable linearity. However, there is a treble boost starting around 8kHz.”
Wharfdale EVO 4.1
AudioScienceReviews gave them a favorable review. I was surprised to read that because ASR is often highly critical.
Polk Reserve 200
AudioScienceReviews also liked these speakers. My same comments as for the EVO 4.1.
Elac Debut Ref DBR62
Again,
AudioScienceReviews like these! In fact, they seemed to like the Elacs a bit better than the EVO or the Polk.
With all of these reviews, you can see reliable measurements of frequency response, impedance responses, and honest measurements of sensitivity. Comparisons among those speakers and predictions of their overall sound quality should be easier. From those you will be able to guess if they are tough to drive for an amp.
Above, I suggested looking in a slightly higher price range than $500-600. These are my two personal picks for very good sounding small bookshelf speakers. Both are sold only by Internet Direct from the manufacturers. You will not find them in any stores or online vendors such as Amazon. I've heard them and highly recommend both. Both Ascend Acoustics and Salk are highly reputable speaker makers and have been in business for quite a few years.
Ascend Acoustics Sierra-1
They have been measured by SoundStage, and look very good.
FWIW, Ascend Acoustics publishes a full set of reliably believable measurements of its speakers. Ascend's measurements look quite similar to those of SoundStage.
Salk SongSurround I
They have not been measured by SoundStage, but are also very good.
Good luck with your search!