I first got the T50RP mk3 and added the Mrspeakers comfort strap, Shure 1540 pads, Mayflower V1 mod. After adjusting the mayflower mod to fix the sound, I thought, that's what i want. The sound was clear, warmer, bass blows original mk3 bass totally away, everything was lifted to these ears. The mayflower mod hulked the mk3 out, and I was on the planar bandwagon. Six months later I found the Hifiman HE500 that only arrived 2 days ago, for a fair upgrade to a bigger driver and great sound. Straight in for 6 hours, sleep, eat, back in 6 hours, then breath. The HE500 just eats the hours. Swap over to the modded mk3, and continue to hear mud come forth from my much beloved mk3, unbelievable bro.
I use laptop>foobar2000>ASIO>FLAC>Ifi iDSD micro
Don't get me wrong, I love my Fostex T50RP mk3 because I spent the time to learn something and it was an awesome experience. However, the king is dead, long live the king. In conclusion, The journey was awesome, but IMHO the HE500 won hands down in all ways and therefor gets my vote as a first time planar to try.
I hope I have kept this jargon lite and rhetoric positive for my fellow novice brethren.