I'd think you could use a broadband transformer. It ought to do OK over the fairly limited bandwidth of the filters.
For 50:2000 you'd need about a 1:6 turns ratio. And the reactance of the high-turns winding ought to be around 2000 ohms or better. That's around 70 uH at 5 MHz.
Ferrite cores should serve.
Or maybe a combination of techniques... 4:1 quadrafilar transformer to give 50:800 then attenuator pad to handle the remaining transformation. Wouldn't have to introduce so much loss, so maybe the level would be high enough for decent dynamic reange.