From this web page;
http://www.udonmap.com/udonthanimaga...animap41_1.jpg
you can see a map of Udon. In square N8, you will see 69 Motel. That is discreet and almost next door to Boyzone on Phon Phisai Road. Gentle is in square O10 and Bandung is in square K12 (very near to what is shown on the map as Boy Boy Pub, but in fact doesn't exist any more there. There is a bar in square M12 called something like Man Bar, but not sure if it's really gay or more for women.
69 Motel is not boutique but is short time place for about Bht300 and has mirrors on the ceilings and fancy bathrooms.
I usualy stay at a hotel on the corner near Robinsons/Central at square O12...it has no name on the map. I like it because it has parking available in front, costs Bht350 a night for aircon room and they don't give a damn who goes in with you. There is a faland restaurant and bar next door called Mojos that is good for all meals. This is a low-key girlie bar and very convenient.
All these places are an easy walk from the main aircon bus terminal and train station.
I am afraid I can't help with boutique...not really my style.