My 2 cents here…
Beyerdynamic recommends an amp that can output 100mW at 600 ohm with a 100 ohm output impedance. For whatever reason, they think that’s the best match for their cans. That’s what their headphone amps are rated for. I don’t necessarily agree. You don’t need that high of output impedance, but the 100mW is probably a good guide. The challenge most budget amps will face is that they may deliver 100 mW into 600 ohms at 1KHz, but they don’t have the power delivery ability to deliver 100mW at 100 Hz and 10000Hz, among others, all at the same time.
You’re in an unfortunate position. The hi-impedance (hi-Z) Senns and Beyers are easy to obtain worldwide but sufficient budget amps for them are not. In the EU, your reasonably priced options are limited. I found the Zen CAN to struggle with bass control for hi-Z headphones, and that was from the balanced output. I have taken heat for that but I stick by it. As iFi’s amp calculator claims they will get plenty loud, but the higher the impedance gets, the more noticeable the lack of control in the bass becomes. See my above comment about power delivery. The Zen CAN has a 5v-2.4A power input, which just doesn’t cut it over the entire frequency range. Your other easily-obtainable amp options in that price range are likely Topping amps - and just…no (IMO). Those have their own problems in power delivery. I found the L30 and A50s to make Beyer headphones go sharp as hell in the treble…way moreso than other amps in the price range, and that only scratches the surface of their issues.
My best advice is pinch your pennies and save until you can land a Lake People G103s at a price you’re comfortable with. That’s a German company and should be relatively easy to get ahold of in the EU. It will also have plenty of power to handle the hi-Z cans, even with the Zen bass boost engaged.
Good luck! Please let us know how it goes.