Hello Tom, welcome here 
Any soundcard with at least two inputs, an S\PDIF (better if coaxial) and a Direct Monitoring function will do an excellent job.
Pay attention that the reported latency is as low as possible (some go as down as 2 ms).
As for headphones, I'd suggest to choose a pair with a good sensitivity (90 dB/V is very low, 120 is very high) and whose impedance is around 10 times or more higher than the soundcard's headphones out's impedance for the most possible linearity.
Welcome, Tom!
To step up from Line 6 device I'd suggest to have a look at Focusrite interfaces.
Good drivers, good documentation, good support.
I recently bought used M-Audio 2626 Profire (firewire) for about 190$ with spdif out/in and 8 chanell preamps. It's working fine with Kemper and SPDIF's - and had a two chanell's out for headphones. As for the headphones- Beyerdynamic DT150 will do the job:D
Thanks for your answers and your welcomes ! I made some researchs on the web and I found : Focusrite Saffire Pro 24 , Focusrite Scarlett 6i6 , Steinberg UR22. The M-audio 2626 is around 450€ in France ... I know the UR22 don't do S\PDIF but does it really make a true difference for the sound? The scarlett seems to be a good deal (in spite of the color 
... ), am I rigth? And my wife Father Christmas thought taking me the Beyerdynamic DT-770 Pro (in 250 Ohm). Is it a good gift or must I divorced ? (I would keep the kemper of course) Thanks for the time you spent helping me.
Thanks for your answers and your welcomes ! I made some researchs on the web and I found : Focusrite Saffire Pro 24 , Focusrite Scarlett 6i6 , Steinberg UR22. The M-audio 2626 is around 450€ in France ... I know the UR22 don't do S\PDIF but does it really make a true difference for the sound? The scarlett seems to be a good deal (in spite of the color
Focusrite Saffire Pro 24 I use myself. Your PC needs to do FireWire though. Scarlett is USB and also recommmended.
Definitely choose an interface with SPDIF capabilities.
I know the UR22 don't do S\PDIF but does it really make a true difference for the sound?
It does make a difference if you want to re-amp. In that case you need 3 inputs and one out in addition to line-outs for monitoring, which you get with S/PDIF in addition to the analogue ports. Sound-wise I doubt you'll be able to hear much difference between the different types of connection. Unbalanced may potentially be vulnerable to noise.
Several people seem to be struggling with the UR22. I don't know if it's hardware, software or operator issues. I have not had problems connecting my KPA to any audio-interface or mixing console as long as I make sure that the output level from one element in my signal chain matches the input sensitivity of the next.
I would suggest Focusrite Scarlet 6i6 or Komplete Audio 6, great interfaces with a lot of inputs/outputs (plus spdif), low latency etc.
Thank you very much for your replies, your detail is very helpfull. The Focusrite Scarlet 6i6 is the best card for me (USB), I'm decided. The question, now, is to find my future headphones. Beyerdynamic DT-990 Pro, DT-880 Pro or Audio Technica ATH-M50? (the DT-770 Pro seems to be tiring for long sessions as a closed headphones and maybe too much bass with it). Have you got some tips for the choice?
Ihave DT150 and they are much more closed and heavy then 770 but I never feel tired with them. You have to walk to the music store and test it:) The closed headphones have one advantage - they are more quiet for the others - and you also do not hear ambient around you.
I can say they are really comfortable.
Another advantage of closed HPs is that there's no return into the mic when you sing or record acoustic instruments and playback a guide track.
I'd strongly suggest you to test HPs with the unit(s) you're going to connect them to. Otherwise it will be like trying a cab with a different amp than the one you're planning to use.
Focusrite Saffire Pro 24 I use myself. Your PC needs to do FireWire though. Scarlett is USB and also recommmended.
Definitely choose an interface with SPDIF capabilities.
Using Scarlett 6i6 on Win7 32-bit, no hassles or anything, stable driver, superb sound for its price. Previously was using Saffire 24 Pro also no problems. As for HP I am using ATH M50. For me big step up from AKG K271 ,I described it in some thread here. AKGs sounded weak. Anyway ATHs can be not comfortable for longer usage.
