OK, I did find one big difference.
I tried amplifying with the new Perpetual G string and found it amplified MUCH BRIGHTER.
My signal chain is: DNA DB-1 bridge wing pickup, straight into the BAM 200 (it has a 10 megohm input !!) then into my MAS 1.6
I had to bring the treble WAY down to sound like the Obligatos. I've seen it mentioned that some people feel Obligatos get 'Lost in the mix'. I doubt that would happen with the Perpetuals.
Since they sound very similar acoustically but much brighter through the piezo I'm next going to try amplifying with the Chuck Israels mic.