My personal preference is for solid state amps, especially in the lower price range. The lower cost tube amps tend to be, to me, tube sound for the sake of tube sound and it gets a little overdone. The solid state sound tends to be the more dynamic and accurate and balanced for me.
The more expensive tube amps are different. The tube sound can be more subltle.
I use an x-head amp. I like it even compared to SS amps that cost more. It does what I need. It can drive my D2000 (25 ohm), Grado (32 ohm), HD600 (300 ohm) and even 600 ohm beyers.
If it was me looking for amps in the under $400 range I'd be looking at things like x-head, Audio-GD C-2c or C-2, and similar.
The HD600 isn't an aggressive rocker. It's reserved. It doesn't deliver the punch and feel needed for rock. It's not the sharpest or quickest on transients. It does sound good though. It's my reference can for what timbre and tone should be. It's a headphone you can listen to for hours without ear fatigue. My HD600 has been delegated my classical music can and also some jazz. Otherwise I've been listening to my D2000 mostly.
D2000 has a fuller sound (it's the bass, but also the midrange gets a little resonance love due to the closed cups). Quicker on the transients than the HD600. Acoustic guitar sounds better. Can rock out better than the HD600 sometimes. But it's recessed in the midrange and that hampers its ability to rock it all the time. A parmetric or a 31 band EQ can fix that. Bump up the midrange where it's recessed and it rocks good.
Grado. Yummm. Blues guitar, rock guitar, jazz instruments, it gives them the love. A good rocker.
A D2000 and Grado combo would cover the rock genres and jazz. The Grado for when you want a more forward sound and the D2000 when you want a fuller sound (bass) and some soundstage.
I'm probably going to be getting a Grado SR325is soon. The Denon Grado combo is tempting as a complementary pair.