Welcome to the world I had some 3.5's and absolutely loved them. They changed the way I look at my hi-fi items. I sold them to a friend and gravely miss their sound. So now, when I buy new hi-fi, I buy and hold. There is very seldom a sell. I must now find an item disappointing to sell. All because of the maggies. I miss Dire Straits and piano music played on my maggies. Very exact, very wonderful. The sub need will depend of what you are listening to for music. For piano, there really isn't as much need. For listening to Radiohead, you might be amazed at what a great sub can add. That's a fairly big room, so you will also get to play with positioning for optimum sound quality. I did mine about 3 feet off the wall slightly towed inward. Positioning can make a huge difference. You will also notice that every different recording will have a different magic volume. I look at the trail you are walking and makes me fondly remember some great speakers I once owned.
As to a pre-amp, I would be tempted to go with guidance from Bryston but ask them if there is a tube option. Perhaps get what they recommend and then try a Schiit Freya. That won't break the bank.
