Having owned both the headphones you've mentioned, I would say that it's a fair enough general assessment that their sound quality may be similar but their sonic signature is different enough to be able to be discerned, but not so different in like comparing a Sennheiser HD with a Grado.
At the end of the day, it does come down to personal preference ( I know you might not want to hear that ) as well as how evolved your music listening experience is. As you can imagine, for some this may be constant and for others, this will chop and change through out your lifetime.
From my own experience with both the K701 and HD650, I have found that the following headphone amps excelled at their respective levels at making these headphones display their potential -
Darkvoice 336i
MAD Ear + purist
Original Master
Meier Cantate
MF X-Cans V3 with Pink Floyd mods ( normal X-Can was powerful, but too dry and harsh sounding )
Even though I have enjoyed and had positive experiences with the above mentioned amps with those headphones, please be mindful that each amp will bring out different strengths/weaknesses in the headphones as well as add to the overall sound the "sonic signature" of the amp itself. Really at the end of the day, there's no substitute to you taking these headphones and demo'ing them with the amps of your choice to see which one your prefer listening to !
Oooh, welcome to Head-fi and sorry for your wallet !
