I actually built a box that does the same as the item shown on the eBay ad... it's just a passive volume control (potentiometer) that goes in the circuit between the preamp-out and poweramp-in connections. I wasn't really happy with the results.
Check the
James Vandenberg mods web site, lots of other info on this amp on that site. I ended up installing the the audio-taper pot in the volume position to replace the linear pot that Fender puts in the amp, and it helped some. The HRD amp is just plain LOUD!