The Zu Omen are extremely efficient 97db speakers. But they do have a higher impedance at 12 ohms. While a higher impedance means that it is very easy for any amp to drive, it also means you are going to get less watts on a normal solid state amp.
Your current
Ragnarok puts 60 watts into 8 ohms, but on the Zu Omen you are likely going to see about 40 watts per channel.
The Schiit Freya has the potential to be an amazing tube preamp if you choose the right tubes. Options can be something fast and high resolution like PSVANE CV181 or something organic and magic like "real" NOS Amperex. The Aegir puts 20 watts into 8 ohms, but you are likely only going to see 15 watts per chanel into your Zu Omen. While it's still not bad considering the efficiency of the speakers, they aren't going to push the volume like the Ragnorak.
The
Elekit TU-8600S
is likely going to be the most organic and engaging solution. But you are looking at only 9 watts per channel with 10% distortion. Be aware that tube amps will also start to saturate at about half rated power, so you really are only going to get about 4 "clean" watts per channel on this thing. It could sound amazing to you, but only at low to mid volume levels.