The DIY subwoofer at the above link is not even ideal, as the woofer is really suited to a ported enclosure, not a sealed one. While it substantially outperforms the HGS10 and slightly outperforms a DD12(yes, it really does), if you put it in the a proper ported enclosure/system, it will more than likely exceed the performance of even a DD15. Though, the DD series has DSP automatic equalizer set-up to correct for some room issues, so they will sound better in this regard as compared to the DIY subwoofer, unless you set up a parametric equalizer on the DIY subwoofer and do manual correction after taking some measurements.
The measurements at both sites are made using a 2 meter ground plane measurement(equal to 1 meter free-field) in an expansive outside open area, and using standardized methods of distortion calculation. So, they are comparable( but I would assume a +/- 1-2 dB error, due to air pressure, temperature, humidity and other environmental conditions since these measurements were done at different times in different places outside ), but you have to watch some of the graph scales, as the ranges may be different, so you have to visually match the X and Y points on the graphs(the distortion graph is one such example). You also have to match the SPL range graphs with the THD graph SPL ranges to find the actual SPL was achieved with the actual distortion level on the graph. Note that the SPL levels in the graphs are essentially 'anechoic'. In an actual room, a minimum of 10 dB gain can be expected at most bass frequencies, and as well, lower frequency extension is increased in actual rooms.
-Chris
