the older qtrons are the best of the bunch, they just all have a pretty noticeable volume boost when you engage the effect, i always had to have a compressor after mine.
Used big box qtron, qtron+, XO qtron+(got the best compliments on this one oddly enough) and a micro, all of them have the same volume boost "feature" even with boost set off. I've gone to the MXR M82, its very much like an FX25 in style of filter, it works best with an active bass, my passives have a hard time getting the filter to open.
But I'm always going to hold onto my Qtrons, weird obsession for them :/