I've had my P/E for almost 15 years and IMO they're actually pretty good as stand-alone monitors. I wouldn't call this the most neutral design ever, but most music sounds very, very good (to my ear) via the Verity P/E. The monitors, as stand alone speakers, have a satisfying octave to octave balance for most music (including IMHO most rock n roll), but the bottom 1 1/2 octaves are MIA.
The bass modules extend the bottom end seamlessly, but add a hump around the top of the lowest octave and (according to my ear and RTA) little real power much below 40ish hz. When mated to subs (I've tried Velodyne SPLs and Rythmiks) the bottom end sounds quite different than using the monitors with Verity's own bass modules. Given any particular sub vs the Verity bass modules, "better or worse" will be probably be all in the ear of the listener.
Some may prefer the subs (and my guess is that good subs will measure better), but I prefer the all Verity combo. As noted above, YMMV.