As the Gen1 xB radio is a Pioneer T18xx series, possibly they can advise on the SSP (Scion Sound Processing) EQ levels for the three sound reproduction pre-sets: Neutral, Hear, and Feel. There also is a selection in the head unit to match the radio to an xB, xA, or tC. I have always used the Scion optional Bazooka bass and am fully please with the sound. But, if you never have used a subwoofer, you should be able to replicate the sound you had before. I defer to the ICE experts on this site for better help with this inquiry.
Good luck with the search to restore the sound. The sacrifices we make for our kids.