okay my fault i didnt mean to be so dramatic. you should be able to use the effects send from the amp through a reverb pedal or a rack mount reverb. something with line level output would probably be the best. i'm not sure if a pedal will play nice with the external power amp input. you can probably try any pedal through the effects send to external power amp in to see if they'll match up. if they do match up there will be no signal loss (ie no volume difference between pedal connected pedal not connected). if your already carrying around a rack for the power amp then you probably have space for a 1u rack effect unit. tc electronic sells a decent unit that does all time based effects (delay, phaser, flanger, chorus, reverb....) for around 150. to use the hrdx this way you will want to run the effects send through reverb unit (hopefully reverb pedal works or rack reverb unit) to the external power amp in. then you will want to pull your power tubes. then it will be safe to pull the speaker and hook it up to your power amp. with the power tubes pulled the output transformer wont need a load to keep it safe. look at it as the samething as doing the preamp tap to the clean channel on a twin. reference dozin website or walds to confirm this. it doesn't matter if its a twin or hrdx.
just my 2 cents the hrdx does a good job of getting jerry tone all by itself. if you use the same settings that are posted on walds web site for jerry's twin it gets damn good and close. the power amp section of the hrdx is 40 watts! i've played outdoor gigs with a 1 x 12 exstention cab where the sound guy was telling me to turn down! this amp is plenty loud for any gig you'll ever play baring getting famous and playing arenas! i guess all i'm saying is save yourself the headache use whatever jbl 120 you flavor 1 or 2 of them and front load your effects. you'll get it close. heres a link to a show from my band where i'm using this set up
http://archive.org/details/SH2011-27-03.flac . i don't sound all that much like jerry but his tone influences me everytime i play. I hope this helps? I'm just tryin to share good knowledge and help some people from ruining good gear. greg...
good to know you got shoes to wear when you find the floor.....