My cabinets are around 120liter (deeper than original) so i added 1 variovent per speaker and it woked well enough - it made bass less dry and deeper without loosing much speed. I fear another variovent might be too much and making those holes in already built cabinets is pain
What i found interesting and valueable about variovents is that they really can lower impedance peak at resonance frequency by a lot. Even thought about trying one on midrange and see if it can work without LCR trap then...