I will agree that amplification is the way to go with the k701s. Using them without a decent amp is like driving a supercar at 30mph. You could be getting so much more of the experience.
I have a Macbook Pro and have listened to the K701s. I actually prefer Sennheisers, but I think that the K701s are excellent. As far as what to spend, you really have to figure out how much is too much for you to spend. What other sources might you be using if any? Are you using only one pair of headphones ro do you have any others? Would you like a solid state amp or tubes?
Personally, I prefer to spend anywhere from the value of my headphones to twice that amount for an amp. Your milage may vary. Both my DAC and Amp are about twice what I paid for my cans, but this is due to several trials with equipment and careful consideration of my needs.
One recommendation I can give you is to try some equipment from
Headroom. You can purchase and audition their merchandise for 30 days before deciding to keep or return it. This is how I got started with amps. I tried the least expensive amp that fit my needs (
HR Total BitHead). When I did a bit more research, I purchased a second DAC/Amp with slightly more features and flexibility (
iBasso D1) and went from there. You can get an idea of your needs once you start using various equipment.
Many moons and many dollars later, I settled on a desktop setup with a
Headroom Ultra Micro DAC and a
Graham Slee Solo amplifier. I couldn't be happier. I even received my
RSA PRedator today. More toys to play with.