OneBlindMouse, I use a pair of these, passive mixcubes to master my tracks... they have what's referred to as a 'useful musical range' (basically 90 Hz to 16 KHz and generally pretty flat), A/B stereo/mono and let me know what my music will sound like on a shit listening system.
They're also good for tracking any bass being pull;ed into that range in a mix.
I thoroughly recommend them, or similar.