Recommend you take a look at the Planar Speaker forum over on Audio Asylum,
here ; the server is down today (Monday 1/4) but hopefully will be back soon. The vast consensus in that forum is for a solid state amp which can deliver high current (typically one that doubles power from 8 ohms to 4 ohms, possibly to 2 ohms). Some (including me) pair the SS amp with a tube pre-amp. Maggie typically demos their speakers with Bryston amps at audio shows, in line with Jallen's post. Others recommend various Class D amps (I use an Audio Research 150.2) but Class D also has a lot of detractors, so that's one that requires a personal listen. But if you look at the Forum you will see quite a few questions regarding recommended amps and the related responses, many more than in a more general forum such as this one.