I use a Schiit Magni/modi combo and vocals sound really balanced, I've never felt like they've been overpowered by the mids or bass imo. I also came from HD 598s, but didn't like them due to buzzing issues. I will say that the highs on the 598 are more amplified in comparison to the mids or bass. On the 650 the Bass and mids are slightly more amplified and the highs are natural and less piercing than the 598s (once again, imo). I can say that the 650's are just simply better in every way when compared to the 598s, more detailed, better positional audio, very full sounding and really pulls you into the music. I hope you will be happy with the Schiit vali and modi paired with the 650, I think the bright signature of the schiit modi and amp mixes well with the warmth of the 650's to produce a wonderful sound!
*edit* Woah, Amazon's price just jumped up $100 for the 650's. I'm glad I snagged mine