I've never heard any Wilson speaker sound lean. I'd agree with the room issue crowd. The same room issues may affect different speakers differently, explaining why Wilsons seem "leaner" than others in the same room.
Other speakers may excite the room differently, because they load it differently. IME, you'll usually find significantly elevated reponse in the half octave above 70hz to 100hz (above the 1/4 wave suckout mentioned by Shadorne) in most rooms. The specific frequency combination of suckout and "hump" differs from speaker to speaker (and placement of the speakers) and determines the perceived nature of bass anamolies for that speaker/room combo. That's why the Wilsons may sound "lean" while some other speaker sounds "thick".
If you don't want to soffit mount your speakers, you might want to think about Hemholz resonators (bass busters) and/or room correction/EQ'd subwoofers. Ironically, by absorbing bass (below app 125hz, the real problem area), bass busters can actually cause some speakers to sound a bit "fuller" as the mid to upper bass comes back into balance.
Good Luck,
Marty