What do you use low pass filters on?

I experiment lots

It's hard for me to know how to treat certain frequencies which aren't reproduced by my system or my actual ears.

if you're talking about your monitoring situation you could always invest in some quality headphones and use those to help pinpoint what you're not hearing so well with the monitors

otherwise its gonna be nothing short of trial and error. You think something seems too bright, low pass it to a point while soloed out to where you notice a difference. Then back it up to a point right before you noticed the difference and see what it sounds like in the mix. It'll take longer but when working with sub par equipment or without experience of how you're hearing (learn your monitors!) it will always take longer. You need to invest at least a little in equip and a LOT on time and frustration until you can listen to something, THINK it sounds too bright but KNOW it really won't be ... or vice versa