I had a Valhalla and loved it. I just sold it to a head-fier who is purchasing it for his 650s. I think the Valhalla will give you a slightly warmer presentation, but not lush. I think Schiit describes the product well when they say it isn't a thick, lush typical tube sound at all, but the tubes do indeed add some of the harmonic distortion warmth. I loved the Valhalla despite selling it (wish I hadn't sold it now in truth), it was never boring, always portrayed the material very musically and always engaging. I briefly heard a set of 650s on my Valhalla, but I couldn't tell much about the pairing as the 650s had some fibres (hairs I think actually) touching the driver that were causing crazy vibrations. That said, the little music I heard was very striking. So much so that I have just today ordered a 650 to warm up my soon to arrive Auditor. I have DT880 600 ohm cans so now I have their compliment in the 650.
All I can tell you is that I listened to a wide variety of music, plenty of rock (Tool, Rush, Sevendust, Pink Floyd etc), Peter Gabriel, Tears for Fears, some Jazz, electronica with the Valhalla and it never missed a beat. I would certainly encourage you to try one. I gambled based on the reviews and opinions I read around here and I have ZERO regrets for having purchased the Valhalla, really a great little amp. I miss it believe me.