Is it safe to go to one of the many small massage parlours in Hong Kong? Or else which are the best affordable spas in Tsim Tsa Tsui? Upmarket chain spas and those in hotels etc will be expensive if that's what you want. That is assuming you are looking for a proper massage. The version with "extra services" is more commonly called just "massage" or sometimes "sauna". Both versions should be easy enough to recognise. Hong Kong is pretty safe, use common sense.
We'll have to assume what the OP has stated for what it is - and unless specified otherwise, this thread would not see the light of day. I hope you realise my remark goes both ways. While looking for massage, you should know what's available, and what you're looking for, so you can get what you want.
Whether it's with "extra service" or not. I couldn't care less whether your remarks goes whichever way. Read what the OP has asked and those mentioned are versions that ordinary folks ask for. Don't try to be a smart aleck and provide other 'extra service' options when it's not mentioned nor asked for. You, of all people, should know full well that this is a family-oriented forum, duh. It's part of knowing what to look for - and to watch out for.
Singapore may be so clean that you can take everything you see offered at face value, Hong Kong isn't. That's why I started my comment 3 with what I guess OP is looking for - commonly called "foot massage" or "reflexology" - and adding some comments about very similar but quite different offers so they can make an informed choice.
I can't help that you see that immediately as "oh, you're trying to steer the OP in that direction". The safety aspects would come to mind when 'Is it safe' as mentioned in the context of 'many small massage parlours' - and not in the context of being offered some specials as what a particular DE has proffered when neither was mentioned, nor asked for.
