Phase correction. The way a classic 4-pole (or multi-pole) filter works when you turn up the resonance is to do with signal processing theory. You have to consider the phase of the signal as a function of frequency when it's passing through your filter. At high resonance, the low frequencies are phase shifted by pi, so they cancel leaving you with no bass. It's how they were designed. The standard virus LPF shouldn't have as much of a severe bass cut as the analogue modelled ones.
Go to DSP related or similar website for a proper understanding of filters. The more you know about electronics, the faster you can make specific sounds, as you'll know what your hardwear is doing, even if it's only a chip rather than discrete circuits.