BX8s are great above 100HZ below this however, they are indistinct and blended shall we say. Better than the BX5s though.
The 8s are excellent first timer speakers but mixing the lo end is pure approximation.
The BX5s are deceptive little critters. They overcompensate the mids so you end up backing them off in the mix. When you take your mix elsewhere they sound dull and lifeless. I compensate by putting EQ across the master buss to lower the mids so I end up slightly pushing the mids so they translate better.
All this said, either model makes for a good first time system. I would lean toward the 8s though.