A few hot tub combos...
The Hotel Nelligan in Old Montreal has very nice suites with heart-shaped hot tubs (perfect for 2, probably large enough for a threesome) but they're not cheap. By far my favorite for an overnighter when I can have a deal during the low seasons.
Also in Old Montreal, the Nelligan brother's Hotel Place D'Armes has also nice suites with hot tubs. The presence of a spa with a hamman (steam bath) is a plus (enjoyed it during a full afternoon and overnighter with my favorite indy, Samy, just before X-Mas)
From my sampling, the hot tubs suites at the Loews Vogue remain the best quality/price ratio in the downtown core. Yes, the elevators require an access card but there is a simple bypass to this : you just ask a second key to be kept in an envelope at the reception and you're date can pick it up without any hassle when she arrives and use the elevator to join you to the room. Did that 3 times, no problema.
The Hotel De La Montagne in front of the Loews has also hot tub suites at lower prices. A bit crapy however, and the tubs are a tight squeeze for two, but if you're on a budget and don't mind outdated upholstery, it's the most SP friendly hotel in town.
Never tried it, but the Sofitel executive suites are said to have nice hot tubs. But they come with a big price tag.
For brief encounters (2-4 hours), the hot tub suites at the various hotels Chablis/Ideal are of course unbeatable.
Lion Heart