The Chen Zhao Kui taiji group (http://www.taijigongfu.com) meets in Harvard Square on Wednesdays but may not be as quiet as you're looking for. Very good, detailed instruction, but also probably more focus on practical tai chi than you're thinking. You don't have to spar, but he encourages you to think about sparring. Would suggest giving it a shot for the attention to detail, though. If you just focus on doing things really right, that's pretty meditative.