Erotic massage studios in Cologne are of the New Age sort. Think more of that sexual tantric therapy stuff than hand job assembly lines. Shiva Tantra Massage (https://www.tantra-shiva.de/) at Greesbergstraße 5 and Ruhepunkt (https://www.ruhepunkt-koeln.de/) at Mühlenstraße 8, should give you an idea of what to expect.
I would have anything of Barry White or Marvin Gaye’s Sexual Healing playing on loop if I would own a sensual massage salon in Cologne. You can get serviced in certain studios offering erotische Massage in Köln, but most people would just go to a brothel instead. Prices are comparable as you need to pay around €150 for an hour of “therapy”.