Is it even possible to run this in the field?
I'm sure it's possible. But is it desirable? According to the specs it runs on AC. Which means you'd either have to plug into AC, or run a DC > AC inverter. Power inverters are generally inefficient and you'd likely need pretty BIG batteries to run this thing through an inverter. But yeah, it can be done. Depends on how much power gear you're willing to carry around in order to run the box.
Attenuators is a given since the lowest gain is 30db.
Attenuators are for reducing the strength of an analog signal. Since the box is a preamp
and ADC and therefore feeds your D8/JB3 a digital signal, you won't need / be able to run attenuators if you're using the ADC in the device - unless you want to open up the box and adjust its gain structure.
The problem as I see it is to power it in the field since it runs on 2x15Volts. Is it even possible to find a battery for it ?
See above re inverter.
The Marenius have a S/P DIF and a AES/EBU digital out. What do I need to connect it with the D8 or the JB3 ?
For your D8, you can run S/PDIF into a 7-pin cable. Check the Archival Info for a thread on 7-pin cable options. Most folks like the Oade or Pro Digital cables.
For the JB3, you need to convert the S/PDIF or AES/EBU to optical S/PDIF. Two devices do this well: Hosa ODL-312 (AES/EBU <-> optical S/PDIF) and Hosa ODL-276 (coax S/PDIF <-> optical S/PDIF).