Well. This thread needs a little update.
I've tried different spas in Seoul: Spy, Superman, Prime, and one that will be called "Isle"
In terms of facilities, Prime stood out the most because it's the cleanest and the service was also good.
I tried Spy twice, and both times left me satisfied, I just don't like how the shower is outside of the treatment room. Also both times were under time.
I met the most handsome masseur at Superman (out of the four facilities), great massage but the ending could have been better.
Isle is more straightforward with their services. It's done at some kind of apartment, which is not modern or clean. What I personally liked about this experience is the masseur looked like a guy I have a crush on haha.
Overall, I would recommend Prime over everything just because of the state of their facility. I've contacted these facilities via messaging apps. Prime and Superman were the most professional in communication.