Hi guys. If you have another pedal in mind, I'm all ears.
I experimented with a tone generator tonight to eliminate the decay variable. The DD-6 cannot loop the "held" sound without a tiny gap.

This is probably no problem for what Boss intended and imagined this being used for, so I can't hold a grudge. The gap is quite noticeable. One interesting thing is that if I left the tone generator on while the sampled audio played back, on each repeat the sound was a different degreeout of phase, and of course an octave up.
Incidentally, I had the new Boomerang III just sitting here too, and it also can't do a perfectly seamless loop.
Looking forward I could either start and stop the hold with the beat, leave silence at the tail or start of the hold, or just lower the effect level so it's less obvious.
Another alternative... MIDI bass pedals and a softsynth.
