As I stated in my earlier post:
you should judge them by the minimum impedance and pick according to that.
Both of your new suggestions have minimum impedances of approximately 4 ohms. See:
Monitor Audio Silver RX6 loudspeaker Measurements | Stereophile.com
Q Acoustics - 2050i Speaker
I would not recommend them, but, again, if you do not play things very loudly, you might be able to get away with it.
Many times, tower speakers with twin woofers use nominally 8 ohm woofers wired in parallel, which presents a nominally 4 ohm load to the amplifier. That is not good if you are wanting to play things loudly with most receivers.
If you look at tower or bookshelf speakers with single woofers, you are less likely to run into this problem, but you should be judging this by the minimum impedance, not by the number of woofers. (Since I have encountered many people online who fail to understand English, even when it is their native language, being less likely to run into a problem does not mean that one will not run into the problem. So ALWAYS examine the minimum impedance before committing.)
Keep in mind, none of this has anything whatsoever to do with the quality of the sound. My advice above is regarding the electrical compatibility of the speakers with your receiver.
Now, with your receiver, you do have preamp outputs for the right and left channels, so you could buy an appropriate separate power amplifier to drive whatever you wanted for those channels. But that will cost a lot, so you would have to really want the low impedance speakers badly for this to be worthwhile.
When buying speakers for a surround system, it is good to remember that the main speakers do not need to have deep bass if you are going to be using a subwoofer. That, after all, is the point of a subwoofer, making it so that you do not need all of the other speakers to be able to reproduce very deep bass. You then need to be careful to set it up properly (which means, in part, that you set the main speakers to "small" and select an appropriate crossover frequency; 80 Hz is often a good choice with bookshelf speakers, but it will depend on the particular model in question).
One thing to keep in mind is that different people have different preferences with speakers, so it is a very good idea for you to go to all of your local audio shops and listen to as many different types of speakers (e.g., ribbon, horn, dome, whatever) as you can find, of as many brands as possible, that are in your price range, to find out what you like and what you don't like. Once you have settled on something you like, you then find out the minimum impedance of it, and see if you have a problem with the receiver you already have. If it is not a problem, great, that is what you buy and you [hopefully] live happily ever after. If it is a problem, you then consider whether there is a similar model that is not a problem, or you consider buying a power amp to drive them.
Usually, one should select one's speakers first, and then buy a receiver or amplifier appropriate for them. But as you already have a receiver, you are more limited in what speakers you can safely buy without buying another amplifier.
Do you have any other equipment other than the receiver already purchased? If so, what is it? And what is your budget for everything you need to buy, and how many channels of speakers do you want? Will you be having more money to spend in the future (so that, perhaps, you could spend all of your current budget on some of the speakers you want, and can wait and buy the speakers for the other channels later on when you have more money to spend)?
