KEF Reference 3 or Raidho X2?

These are very different speakers in terms of power requirements, room requirement, sound profile and soundstage presentation. 

Kef is a classic "cone and dome" speaker.  The image is presented to you as if the performance is in front of you.  Raidho is a ribbon and delivers a more immersive sound.  The sound reflects off the walls to create the soundstage and it feels like it surrounds you.  Neither is good or bad.  Just different. 

The Kef's are a little brighter and require careful toeing.  Tonally, your equipment should work well unless your room is under-damped.  The Raidho's are more natural and less sensitive to what they are matched with.  It will be reasonably natural unless your room is overdamped in which case it will drift dark.

From a power requirement, the Raidho's are easy to drive and work well with tube gear.  In a normal size room, 50-75w is plenty.  Kef's like a bit of power and are a touch more difficult to drive.  I like 175w to drive them.  

For room, the Raidho's NEED 3' behind them from the wall, minimum and 9' of space between them.  If they don't have that, the soundstage will not come together properly.  Proximity to the sidewall is unimportant. Kef's should not be as difficult to place and will be more flexible.  

You will need a subwoofer with the Raidhos.  You may or may not want one with the Kefs as their bass response is much better.

Raidho pricing is a topic of debate and is better left as a conversation between you and your dealer.  

FYI..I am a Raidho dealer.  I am not a Kef dealer.  

The difference between X2 and XT2 is simple.  X2 uses a ceramic driver.  XT2 uses a ceramic and titanium driver.  The titanium driver is clearer and will deliver more detail.  
Whether it is worth the upcharge is debatable and is driven by personal
philosophy and quality of equipment you have driving them.  Are you focused on ans listening for small details?  Is your equipment equally brilliant?
This will show up in inner detail with massing of strings in classical or separation of a drum roll.  
You mention an ARC preamp whic will be fine.  Not sure what power amp and tubes you are using along with cabling.  This is a level of speaker where cables matter and the difference between speaker will be more pronounced with a good cable.  DAC also matters here as it can be limiting. 

I have not seen an issue with a sagging tweeter.  The first I had ever heard of this was in another thread. 

That Marantz unit is excellent.  It will more than do the job.
Regarding the amp, this is a tough one.  I have never heard that amp.  I tried researching it and available data is limited so my POV is based strictly on spec’s and is worth exactly what you paid for it compared to your own experience or someone who actually knows the amp.

This a relatively low wattage class AB.  If you are in a larger space or listen at higher volume, I would be concerned about how much you are taxing this amp.  I would rather it be Class A where output is constant and then, running volume at 90% would be no big deal.  

Secondarily, I know nothing about the internal parts used and the quality of the amplification.  Price makes me wonder but I have heard awesome, modestly priced amps and others that are awful.  Quality of transformer and caps is key here.  

Because of my recent experience with the Canor AI 1.10, I have a very favorable opinion of EHX KT88s.  They do a careful match and they can be brilliant.  
In the end, only you can know if this amp is up to the task. 
This amp may work with the Raidho’s.  I would not use this for the KEF’s unless you are in a small space.