Massdrop has sold over 20,000 ( there's a red one too ) of the K7xx so they're doing something right. They need an amp/DAC as much as the 600/650 do though.
The 598s are good cans, even really good but they aren't in the same league as the 6xx series. However as you're finding the 6xx series is expensive to get into. The 650s are considered some of the best scaling headphones on the market. So if you have a $1000 amp they'll reward it. On the flipside my $50 Fiio Q1 will drive them.
HD 598 - Big soundstage very little bass. Overall good for gaming and music. Don't especially need an amp/DAC but will reward them.
HD 600 - Intimate soundstage, neutral, slightly mid forward with amazing detail. Bass but not much. Rock, classical, folk, jazz ect. ( my personal favorites, when they sound good they sound GREAT ) There's a reason these have been on the market since 1993.
HD 650 -Wider soundstage than the 600, warmer with more bass extension. More laid back. Still mid forward with ALMOST the same ( and different ) detail as the 600s. More forgiving of bad recordings than the 600s. Work with a wider variety of music. Same as above but add newer pop, EDM, hip hop etc. The 650 was Sennheiser's flagship for years.