The BL2 is less precise than the BL5. The motor that moves the diaphragm of the speaker (the throw of the speaker) is not as fast and precise as the BL5s. Therefore, in portions of a soundtrack where there's a lot of information, the BL2 will interpret this as one WHUMP, where the BL5s will properly decode it, to let you hear what you would hear in a good cinema, with a starting blast, the explosion itself, the aftermath (exaggerating a little, but it's the principle).
Just to move a similar amount of air (though not as efficiently as a pair of BL5s), you would probably need eight BL2s.
I think your dealer failed to do one of the following:
1. Correctly specify where the LFE signal should go, in a setup without a separate subwoofer and with BL5s as front speakers.
2. Correctly identify which version of the movie's soundtrack should be played.
3. Correctly specify the boundaries of the LFE signal (the crossover point) in the processing.
4. Had Dynamic Response limitation engaged, when the BL5s were connected. Probably "night listening" mode, without bass.
Anyone disappointed by the BL5s bass response in movies (when properly set up) would probably need to wire individual sticks of dynamite to their sound system and set those off at each explosion. 😁