get a pair of hi-quality subs. that way ewe don't have to compromise the thiel's imaging/soundstaging to get better bass. whil i personally prefer passive subs w/an outboard electric x-over & separate amplification, i'm prolly in the minority here, as most prefer active subs w/built-in electronics. no matter, get subs! :>) while i no longer use 'em in my main system, my thiel 3.5's, flat down to 20hz, benefitted from subs...
good luck, doug s.