Many thanks to SHJohn for a guide to finding prospective massage parlours through Dianping.  But also a word of caution as described below.

Using the guide, I identified about 2-3 anmo (massage) places in the JingAn area with potential for sketchiness and then decided to visit 果东JK*剧情换装*SPA会所.   The Dianping profile had the requisite hints, e.g., some photos of sexy therapists, a photo that said "for adults only" and some vague reviews.   I booked a timeslot via Dianping and prepaid a 499 CNY session (discounted from the "list price" of 899 CNY).

2 minutes after making the online booking, I received a call from a lady at the spa, saying that the address on Dianping is an old store in the process of remodeling and could I visit a different location instead.   The new address was only about a 5 minute walk, so I didn't mind.  And, in any event, I figured this address change seems consistent for the operating procedure of a sketchy parlour.

I arrived at the new address, which turned out to look rather credible.   The storefront said "SPA" and the interior (front desk area, facilities) looked modern and legit.  I checked in and a staff member escorted me to an interior room which was nicely decorated but darkly lit.  Then informed me that they had three options, two different 60 minute sessions and a special 90 minute session, at different price points from 999 CNY to 1599 CNY.   I inquired about my 499 CNY prepaid package, and she said that was only for very basic service (??!).  Still curious, I elected for the 999 CNY package and agreed to pay the difference.   I was already there, and very curious what was available.

A few minutes later, there was a knock on the door and my therapist came in wearing a modest black dress.  Nice face, although slightly on the heavy side.  She gave me a pair of shorts to change into and then took me to a common shower area so I could shower before the massage.   The shower room was nice -- it looked modern and new, with many clean towels and toiletries.  In my experience the sketchy places are rarely this nice, so I was uncertain what to expect.

After going back to the room, the session started with me lying face down and the therapist applying oil and giving me a decent massage for about 30 minutes.  She was very talkative, which is fine for me (I can speak Mandarin).   In our chat, she said there were all types of clientele (local and foreign) and also shared that different clients had different interests, such as S&M, and this spa could cater to all sorts of needs.  I was now thinking that I had come to the right place.....

Right after the flip, the therapist told me her manager needed to come in to share some service add-ons with me.   I figured, why not, I am now in the mood for an elevation in fun.  The manager came in and said there were different service/play levels available, and the best way to unlock these would be to buy a prepaid membership of 6k+ CNY.   The membership would give me access to premium sessions like Japanese-style, 2 on 1 style and S&M/cosplay.   I told her no, that this was my first visit and only if this service was good enough, then I would consider buying a membership.   She then tried to convince me to buy a trial membership of 2k CNY, which would at least give me a good one-time experience, and again I refused saying I had already committed to 999 CNY.  I told the manager to let my therapist finish the rest of the 60 minute session and then we could discuss further.  

The remaining 30 minutes was uneventful, still a veggie massage but clearly the therapist was disappointed.  You could hear it in her voice.   She told me she liked me and was interested in providing more service if I purchased the add-on, but clearly whatever mood I had was already disrupted.  I asked her what was meaning of "more service" and she was very evasive -- "you know what I mean" -- which just validated my belief that this was just a "bait and switch" place.

When the session ended, I went back to the front desk to pay.  The manager never came out to greet me -- I guess she knew I was a lost cause.   I had been swindled by a similar scheme many years ago, so I considered it a "win" to have left with a decent veggie massage and only 999 CNY out of pocket.   

Sharing my experience so you can avoid a similar one...

dude if it was me i would just deck out of there when the massage i done. no rub no pay