One person's opinion.
Onkyo: Sold it for years. Many people love them, I thought they were OK but not much different that anything else, unless your talking their higher end power amps, which are excellent.
Marantz: Pre 1978 great, but you do pay for it. Whether it's worth it or not is up for debate. I have a few nice pieces but I didn't pay through the nose for any of them. The older the better. After 1980, wouldn't touch with a ten foot pole, until you get into the 2000's. Not familiar with the new gear but has a good reputation.
Rotel: Never owned any personally but any I've heard sounded great, older and newer. Seems to be a great value for the dollar.
NAD: Sound quality excellent. Build quality not so much. Some of the newer pieces are supposed to be much better. Sold it in the eighties. Probably the best sounding and least reliable brand we sold.
NAD established themselves in the early 80's by making great performing amps with great designs but using cheap parts. This has worked for them, but has given them a reputation that is hard to shake. If I were looking for a brand new amp, I would look at NAD first because their new stuff is as reliable as any other. I would look at Rotel second. I've always liked Rotel equipment. 3rd would be Onkyo. This is just my opinion, but the new Marantz equipment always seems overpriced. I don't know anything about Emotiva

I have a love / hate relationship with NAD. I have owned 5 pieces and 2 of them failed just out of warranty. I also recently purchased a new NAD C356BEE integrated amp which developed issues within the first couple of days.
It would shut off after power up.What was the issue with your C356BEE?
Owned all of the above (both old and new in 3) except Emotiva and I'd rank them:
Rotel
Marantz
Nad
Onkyo
The Rotel has what I can best describe as lush sound. Just full of dynamics. Both vintage and modern Marantz pieces I've had have been superb, and I currently am using a Marantz AVR in my home theatre system. Nad have a nice sound, but I have also been a victim of quality issues, found sound quality variable in 1980's models (some stellar and some ok), and was unimpressed with the last AVR I had from them. And hey, Onkyo is good too, though is a step down from the other 3. Interested to hear how the Emotiva stacks up.
That being said, I listened to a modern Marantz integrated in a sound room the other day, and was tempted to peel 900 bills out of my pocket for it.