Unfortunately I won't be able to directly compare Hugo2 vs. Hugo2 with CMA600i.
I did have a Hugo2 for a few weeks with me last year, but I had different headphones back then and didn't have the CMA600i yet.
Now I am running the Qutest into the CMA600i and use the balanced output of the amp. The Qutest is essentially the desktop version of the Hugo2.
There will be arguments on both sides, which one is better: Hugo2 alone or Hugo2 with an amp. Purists say for the matter of transparency you don't need an amp with Hugo2. Some others say they get a more enjoyable sound with an external amp even though they may loose a bit of transparency. Especially with hard to drive headphones the additional power of an external amp can come useful. It often adds body, texture and bass heft to the sound. On the other hand I remember one member said he can hardly hear a difference between Hugo2 alone or H2 connected to the Auralic Taurus which is a similarly neutral(ish) amp to the CMA600i.
Working from memory here, Hugo2 alone sounded extremely detailed (and transparent) to me, but I imagine it can sound a bit thin with certain harder to drive headphones, especially planars. All I can say about my Qutest/CMA600i combo is that I really enjoy this match. Qutest is extremely detailed, has a pitch black background, beautiful cohesiveness and 3D space with great depth, air and separation. The 600i adds awesome sweetness and smoothness to the sound, powers up the low-end especially with my planar headphones.
I imagine, adding the 600i to the H2 is technically probably unnecessary. However if you'd like a different taste of sound, a sweeter and smoother texture, which you may or may not prefer to the H2 alone, it might worth it for you. Only your ears can decide, nobody else will be able to tell you.
You can always sell the 600i later on if you don't find it beneficial. Or if you only use the H2 as a desktop DAC, and you love the taste of the 600i, you can swap the H2 for a Qutest to save money.
If you have planar headphones, I'd definitely encourage you to try the 600i as planar headphones love current more than voltage and the 600i has a lot of current.
I wouldn't worry about feeding SE DAC and using the balanced output of the 600i, the sound will be awesome. At the end of the day it is the balanced circuitry/implementation of the amp that matters and gives you the sound you want (or don't want).
I hope this is somewhat helpful.