I still use my old 4B-SST² I bought for previous 3.6 with my current 20.7 Magnepans. The sound is great, with the other upgrades, Duplex, power cords, conditioning.
Also, do you use the resistors? The midrange resistor is really is more to adjust the tone..(You can know right off when it is too large a value as it muffles up the sound a little, too low a value the sound is a little thin.) With a careful selection of different size resistors, you can really improve the sound and match the mids sound to the woofer!. I bought a pile of Mundorf ($1.79 each or so) from Madisound) 10 watt MOX 0.47 0.68 1.0 1.5 2.2 I bought a few extra so I could mix and match to make in-between sizes. Once I found the size that really made the sound perfect, I bought expensive Duelund from Parts Connexion at half price. The tweeter is more of an attenuation. I found making a choke woth ordinary wire and a dowel makes a great choke, add the choke and a smaller resistor. (I use 0.75 Duelund) for the treble.
I would say it all depends on how loud you play music?
I rarely go over 75dB at listening position. If you are hitting 90dB regularly.. You probably could use 7's. I would think you would know if your amp is running out of juice.
The other side of it is the control. The 7's could give you more control over the bass.
First: do you have Mye Stands? They would do as much or more for your bass as going to 7's for a lot less money.
I would definitely recommend Mye Stands.
The better amp??? hard to say.