whats first order filter on spkr's cross over ?

You may find this link useful:
http://www.thielaudio.com/THIEL_Web/Pages/faqtimephase.html#anchor564257

It means that the slope rolls off frequency at 6db per octave. For example, if you take 100hz frequency with a 6db filter, it will be down 6db at 50hz and 12db at 25hz(2 octaves shown here)
6db filters create less phase shift and as a consequence are considered to affect harmonic structure less than steeper sloped filters such as 12db or 18db, etc. All time and phase coherent speakers use a 6db slope filter in the crossover along with offsetting the drivers for temporal alignment(time alignment.) I cannot attest to the bass alone because there's more to the design than just the crossover.