Ultimately comes down to how much are you willing to/are able to pay for an incremental step up in quality? As with most of the finer things in life... the law of diminishing returns will apply. And IMHO, for headphones and amps/dac it begins around the ~$200-300 mark... where the cost for performance starts that exponential climb.
If you're not interested in that climb, likely, the HD600 (cant' say for magni/modi haven't heard) will be more than enough for the rest of your life.
P.S. Congrats Slayerming2... I believe you've made an EXCELLENT choice with the HD600s