Thanks Tim,
I'm not using a high pass filter on the entire mix but I used them to some extent on most of the instruments (bass guitar and kick included). I think I high passed the kick at 60hz (but also boosted +6db at 55hz) and the bass guitar at around 60-80hz. This made them both a little clearer and less 'rumbly'. I think I just need to clean up the bass guitar and bring it up in volume a bit, but I'm worried about it making things muddy. I have a subwoofer on my system (which can't be turned off) so it might make the bass a little higher than it really is.
Chris