The Bryston 4B-SST² I own with my Magnepan 3.6 is totally all I need.
I have plans to buy Magnepan 20.7 and still use the Bryston 4B-SST² I currently own.
PRevious to the 4B-SST² I had a 50 watt per channel, and really wanted more power so the power hungry Magnepan 3.6 would be totally happy.
As for you.. I would say what you need depends on how you play music.
If you constantly (or even ever) play hard Rock at 110dB plus.. in a really big room.. and it's a party full of people.. you might need the 7's. (or a 14B-SST³)
If you play just for yourself in an average/big room, and usually never at ear splitting levels. The 4B-SST³ (I usually play music at 60dB to 80dB "C" weighted at the listening chair.. apps for dB meter are available for your smartphone and work great. see what you like for dB levels?)
The 'idea' of a pair of mono amps is your friends idea.. I would really skip it and buy the 4B-SST³
One point in favor of a pair of amps is being able to have each amp right at the back of each speaker. Which in some situations makes a lot of sense.
If I had money and wanted a 'pair' of amps, I would buy the 28B-SST³ and just be done with it!
And get the 19" and handles! moving them would be way easier with handles...)