I tried various amps, both solid state and tube and quickly found the big VTL 750 was was making better sound. In fact, when the VTL 750 was on ribbons that is where the best sound was coming from. Reverse and put the VTL 750 on bottom and it was better sound than top.
In other words if the woofers had the big tubes it sounded better than the ribbons (you could hear the mismatch). Reversing got the same effect. Believe me, I was trying desperately to not buy two pair of big tube amps but that's what it took to get the Dali world class.
My (now) Focal Grande EM are certainly better and simpler set up but I still have one guy in my group that misses my Dali Megalines with four VTL 750 and twin JL Audio subs.
Solid state might exercise a little better grip on the woofers but they are only approx. 6” and very quick and controlled already. They are more in need of what tubes do.
I no longer own VTL but they were a great amp, especially for that time. I ran VTL 750 on my Soundlabs, then the Dali Megaline and for the first year with big Focal Grande EM.
The new VTL Siegfied is better than the 750, and so is the Allnic M3000, Allnic M5000, ARC Ref 250 and probably other more current design tube amps. I would go for at least a couple of hundred watts on each of the four positions needed to operate the Megaline.
Last, the crossover that Tom Tutay builds is a huge upgrade over the stock one but does so without risk to the Dali design. The crossover point is indeed fixed but the flange point of Tutay crossover allows varying boost / cut of low frequencies via silver contact volume control switch, allowing you to tailor the sound to your room and compensate for distance from wall and corners.
Remember, there are capacitors in the base of the Dali Megaline that the signal passes through to make the crossover operate properly. When you choose different EQ or crossover from factory specification you risk damage to the tweeter.
The 7 foot tweeter already dips below 1000 HZ which is very low frequency for a ribbon to have to operate. I believe if you put in enough power and operate at frequencies below what it was designed for, you might be inviting problems.
If you follow your own path to crossover points and EQ, I suggest you order replacement ribbons from Dali while they are still available. You probably already know this model is discontinued and is why I sold mine and went to Focal.