So, FYI the Bass Buddy has gotten a lot of love in this forum, meaning there are a bunch of threads for you to read.
As far as connecting it directly to your soundcard, generally the answer is yes, but you have to ask yourself: what does that mean? What are the requirements of your soundcard? What devices are intended to be used with it? What makes one device compatible and another
not compatible? If you can answer those questions, then you'll be able to figure out the Bass Buddy as well as any other device you might want to connect to your computer.
I use the Beyerdynamic phones (the 770 and 990), and they are superb. With the BB and other headphone amps that specify lower impedance values, the result is that the Beyers are not very loud. They are perfectly adequate, for non-ear-injuring play-along levels; but if you want ear-blasting loud bass from your phones, you need a different set with lower impedance.
There's an explanation of speaker impedance in the FAQ, so check that out.
"Volume creep" is where you listen to a steady stream of loud sounds/music, and your ears become desensitized after a while, so it doesn't sound as loud anymore. So then you turn up the volume knob on your amp or w/ever, and it sounds as loud as you like... for a while... then your ears become desensitized more, and they turn down the volume going to your brain again, and again it doesn't sound so loud anymore. So you turn up the volume knob on your amp...
So at the start of your gig you have your volume at like "3" (figuratively), and by the end of the gig it is at "10". It sounds fine to you at that moment, but your ears are ringing, and they have probably been permanently damaged.
Solution: ear plugs.