Using subwoofers HF crossover

I am gonna be demo-ing new speakers within a week, if that fails to give me a substantial improvement I am gonna keep my Thiels and consider purchasing a subwoofer for my system (Thiel CS1.0, Marantz 7200 amp, 6004 cd player in a 12x14 mancave room), which I know will give my bass-shy Thiels and system the needed change. Some subs have a LF input and output that I can loop my signal from the pre-amp stage into to sub and back to the amp. Using this crossover prevents the main amp from using its power to drive my main speakers for frequencies below the selected crossover point. My question is, will this advantage outweigh the possible degradation of the source signal and lose potential sound quality in the mid's and high's? I ask this because in selecting a Sub some have this as a feature others do not. Some people say just run the sub in conjunction with the main system. set the subs low frequency pass to match the rolloff of your mains and let the main amp and speakers do there normal work.