In terms of people questioning Sennheiser's expertise at making headphone amps, even ignoring the HEV90, it's an odd sentiment.
Sennheiser is a major corporation who have been making headphones and microphones for professional and consumer markets for a long time. Any expertise they don't already have in house they have the resources to hire. Some of the companies people have glowingly quoted as being the true experts at headphone amp design are very small companies with self taught DIYer at the helm.
Some of those companies make great amps, some make questionable products that have gained favour in a small online community through customer reviews and group think. Not going to say which I think are which, as I'm not even trying to denigrate anyone, but lets just be realistic here.
If Sennheiser want to make a headphone amp, they are going to use credentialed electrical engineers with experience of working for other renowned corporations on similar products. They are going to have all the tools to test and measure and perfect the product they are making. Sennheiser is not a guy sketching out a DIY amp idea in his garage, working by trial and error and judging success by ear.