They are based on my taste and preference. 1. Mjolnir is very detailed, clean and natural vs. Crack+SB is very enjoyable and musical, also pretty clean. 2. Crack: Very musical and pretty warm vs. Valhalla is bright (not as bright as the SS amps) but pairs well with the HD6503. Lyr is warm, punchy, and dynamics 4. Vali does the job If you like the warm sounding amps, Woo WA3 is also a good choice, IMO.