The Glenn OTL has a more romantic sound compared to the Euforia. Instruments hold more weight, images aren't as sharp as other amps (fuzzy around the edges), mid bass hits harder, and has a big sound stage (side/side and front/back). Tubes really change the sound of the GOTL.
The Euforia has a more solid state like presentation. Sharper outline, faster, with more extension. The amp is clearer and you don't have background noise (if this bothers you) compared to the GOTL. It's all about synergy really. I wouldn't pair the HD650 with the GOTL but that headphone sounds amazing on the Euforia. I would definitely pair ZMF Auteurs with the GOTL but think it's ok with the Euforia.
On the other hand, I would pair the Verite Open, Auteurs, and HD650 with the ZMF Pendant any day of the week.