I've had the business cards of a great jimjilbang cluttering my desk for months now, so I'm just going to scan and post the info and let you know that this place is pretty sweet: the clay kilns out back are a wonder, and the coal roasters where you can buy rice cakes, potatoes, sweet potatoes and corn, and then roast them yourself, are AWESOME.
You should go there.
hanbangland.co.kr. No promises the website will be useful.
It's between the Shinchon/Hongdae area and the Jongno area, and the masseurs are hella strong.