go cheap - $50 or $60 gets you a Rolls PM350b "Personal Monitor Mixer Plus". I use mine alot.. three inputs (line, instrument, and mic) and 1/4" and 1/8" outputs (can be used together). The inputs are switchable stereo/mono.
I run my PC into the "line" and plug my bass into the "instrument".. can crank music and/or bass all night and the wife doesn't say a word. Plus, you have seperate volumes for each input so you can mix it if you're trying to learn a song off cd/mp3.
Headphones late at night, PC speakers (monsoon) anytime else and it's fine. Besides, even when I'm not playing bass it's a handy PC volume control.
Doesn't allow for tone control really but if your bass sounds okay direct it's more than adequate for practice. For 50 or 60 big ones why not? It also doubles up as a useful tool for home recording.
Just my 3 coins..