Mozvz is right, consider keeping the 350 to drive the 800s for 2 channel and add a multichannel amp or avr with a bypass for ht.
I have a similar setup, Dali 800s (mk1) for fronts, 200 center and 400s as rears, servo 15 sub.
Currently using a denon 5803 AVR receiver (7x170watt) and added a conrad johnson Met1 6 channel tube preamp,,, huge improvement! In fact liked it so much have a cj Met150 hybrid 5 channel amp on order from Spearit Sound to replace the denon with dedicated amp. Wanted the Odeon but its now 12k$ and the cj multi channel stuff costs much less and sounds really great. You can get the cj met1 and 150 pre and amp both for 8k$ here on agon brand new with 3yr warranty, lass than half price of list because they are discontinuing their multi channel line.
Im no expert, but it is clear to me, when flipping back and forth from 2 channel to multi, that the 800s cleary need more power to drive them than the 200 or 400s. I will know in about a week if the 150 can handle the 800s. But in about 5 years, when Ive paid for this stuff, I will add a X350.5 or a cj 350 for the fronts. So I would keep your X350 and build from there.
Dalis are great aren't they :)