Now, you didn't give the thickness of the horisontal sheets (with holes) used for bracing - nevertheless, I doubt that you lost more volume than a few liters... It's not difficult to calculate the total volume of the added material - but I think you might be surprised about how little space it really takes up.

One explanation could be that the holes in the bracing sheets are too few or small to permit the free flow of air at the basic resonance - that could mute the bass quite a bit. A more 'gloomy' explanation could be that the bracing actually did it's job, & the lack of box resonances in the mid/upper bass area have simply given you a more clean bass! But also less of it cos the resonances are gone... If this is the case, you might need to retune the box to get more bass.

The problem here is, that a reduction of the volume (as a result of the added material) would normally lead to a higher total 'Q' for the box - which should give MORE bass, in terms of perceived sound.