Just use a low pass filter. And for finding keys, I use mixed in key. Gives you tempo and key. Knowing your scales and being able to figure it out yourself by ear is invaluable, but mixed in key is a time saver.For the most part, I do know my scales but its just hearing it and knowing that it is in a certain key is difficult for me. It's something I have to work on though.
Maybe I'm not doing it right, but I can never completely just key in on the bass when filtering it out. You're simply using an EQ to filter it out right?