What is the best loudspeaker for a small room?

Agreed, not that small of a room when you consider 15x20 average. However, it all depends on how the room is configured and what furnishings. After downsizing I have a similar size room and I can tell you it has been a challenge. After trying numerous speakers I ended up with Revel M22 and a REL subwoofer.

The best speaker in the world is the speaker you like the best, period.
Next time you might ask what are some of your favorite speakers for a medium sized room?
I missed you budget but some used Sophias by Wilson would be nice if you have the electronics for them.
Do you like any particular type of sound? Some might say Harbeth if you like that British type sound? You understand I am sure.

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I would go for a mid sized floor stander or a bigger sized monitor on a stand. I've messed around with all kinds of speakers in my bedroom that is about the same size and a mid sized floor stander will give a lot of bass at least in my room it does. You have some darn good electronics so I'm sure you have a pretty decent budget. You prob could make a set of Wilson Sophias work or how about revel studio? Or a Dynaudio? I'm not sure if what is known to match well with Pass Labs but I'd start there and don't be afraid to get something mid sized. Your room is not that small.

I aggree with the above. DOn't get speakers that are to big but get ones that fit the room. Sounds like you really need to walk into some dealers and listen a bit and see waht you like once you give us some feedback we can recomend similar speakers to your taste. Some dealers will even let you take speakers home to try.