SubsOnly is a utility plugin to use with Voxengo SPAN, in Density Mode. That will give you the best ‘VU style’, sense-the-loudness-of-the-track, metering, and when you put one of these utilities in front of SPAN, it’s calibrated so equal levels will give you appropriate amounts of extreme subs. If the levels are out of whack, your balances are also out of whack. (Not all mixes will produce the subsonic frequencies SubsOnly tests for, so it’s okay if they’re not present, but they should not be blowing away the outputs)
SubsOnly is a very simple algorithm: it’s just twenty-six IIR lowpasses in series. This produces an outrageous but very primitive lowpass, so you can also use it as an audio filter. It should sound a lot like listening to your mix from an adjacent house, or something, and you can use it to see what instruments are taking up subsonic space and get a sense of how the beat comes across at a distance.
This algorithm is available in Ditherbox, for convenience. That way you can switch it in for mix checking, without having to load extra plugins.