Mike on Headfonia said that HD598 gets many benefits from mid or high end amplifiers, but be aware he stated that between HD598s and HD650s, the HD650s can up-scale more than HD598s. He used Zana Deux as an reference amplifier. To sum up, if you use high-end tube amplifiers you can find out that HD598s could be too mellow and suitable only for some genres.
Someone said that HD598s do not need separate or inexpensive amps to bring out of them. I totally disagree, I and many other people think by using good separated amplifiers to hear all of their potentials. For me, as I visited a audio store near my house and tried to use my HD598 with many from low to high amplifiers, some amps can make HD598 sings beautifully but some amps do not. By saving money I bought a Fiio E10 but I had tried a Fiio E17 and found out E17 is better than E10 clearly. The instrument separation, the mid, the bass, the transparent are surely better than E10. My E10 however I found it is muddy, not clear, gets harsh with some songs and little bright to my ear.
I ordered a C421 OPA2227 to pair it with my HD598 and E10 (using E10 as a DAC only). Hopefully the result will be better, HD598 on my memory should be a little warm and smooth. Next I will be saving money to buy a better DAC such as Dragonfly for example.