Personally I think you're thinking about this the wrong way round.
You already have speakers that are of an excellent quality: speakers that are in no way reaching their full potential with your current amplifier.
I would always recommend where possible in a surround system, using identical drivers across all speakers. So if you want to improve stereo sound by beefing up your main left and right channels, I would go for the Monitor Audio R270's which will help the midrange in your system while crucially maintaining the identical drivers across all speakers. Lobbing some Q Acoustics speakers into your system won't have the same tonal balance across all speakers and could do more harm than good.
The same is true with Monitor Audio Silvers: I've found that having owned a number of different Monitor Audio speakers (Radius, Gold, Apex), that they DO differ slightly in tonal balance. So if you want to retain your R90's as surround speakers, which is not a bad idea, then I would stick with Radius for your main left and right channels. Otherwise you will likely find that certain channels, despite whatever level you set them at, will still stand proud too much because they have a little more zing than other speakers in your set-up. This is not what you want to do in a surround system to achieve a realistic sound field.
Also, do not underestimate the quality of the Radius speakers. The weak link in your system at the moment is the amplifier, not the speakers. By all means upgrade the main left and right speakers as the R270's are more capable than the R90's, especially for 2-channel sources. But following that, I'd take a good look at upgrading your amplifier so you can really hear what the Radiuses are capable of.