Logic will tell you that the B25 has more bass than B15, and that's it. But, no the B25 is superior in other aspects like soundstage, imaging, and neutrality. You need to compare them side by side to truly appreciate the difference.
When using my B25's, instead of my T45's, in my HT system I have them aproximately no more than 12" from the wall, and they are not boomy at all. The fact that they are front ported is a big help. My family room is 14' x 17' with 12' ceiling (all concrete no major acoustical treatments), maybe in a smaller room they sound boomier, but not on mine.