In this case the lfe.wav would have to be disregarded, and a new one created. This would be done by duplicating the two surround channels, summing them to mono, applying the low-pass filter, and raising the gain by the appropriate amount. In order to know the amount by which to boost the LFE, it is important to know first whether the process of summing to mono keeps the level at unity, or causes a gain increase of its own. In the first case, the +17 dB increase would be correct; but if the resulting mono is 6 dB louder, then you'd only raise it by 11. (I know it's confusing; it took me a while to wrap my head around all this.)

If you like, I can take care of all that and send you the results—all I'd need is the 2.0 PCM (synched to your video) to line it up with.