This is Audio-GD's specialty. Not only do they excel with all-in-one, but their amps all thrive with planars. See NFB-29 for single ended or NFB-28 for balanced.
What happens to the Gustard? Thought you like it with your HE-500...
I had to sell it to save room. Moving into an apartment and desk space is at a premium... Ugh.
I still think it's a fantastic amp.
It might be only me, but I found Audio-GD stuff a bit too polite for my tastes
I had the GF TubeDAC 11 for a bit. It is a nice little all-in-one unit but the amp section wasn't fantastic. I'd use it more for the DAC than the amp. I don't think it would drive the HE500's the best.