I own a hd 595 not a 598 but I' ve listened to one and the he 400 is a nice upgrade
The 400's have a better soundstage some nicer trebles (imo) and especially a better low end
Mad dogs could be a nice upgrade also but imo the he 400...
Yes you only need 2 opamps for the built in amp and only 1 if you use just the rca out (amp)
Myself using a lake people g100 I've only changed 1 opamp and I have LME 49990's
K701 are very good headphones
the stx has a good built in amp you may start with that
The headphones make the biggest differences in term of sound quality, the amp is like 20 % of the sound
But the k701 are known to need current to get better bass, some thing like...