Assuming you don't want to live with just two digital rack spaces (like me
) ... here's what I would consider doing.
The H&K is a half-space unit, right? So take that, and get yourself one of the small Class D heads that you can tap into the power amp of through the effects loop (I used an Acoustic Image Focus 2R but I also have a Genz-Benz Shuttle, which is actually a 600-watt bass head that I tried out with my old JBL cabinet, and it sounded great, too). These little heads weigh about 6 pounds and are about the size of a book. Now you have your preamp and power amp in a tiny, lightweight compact package with a ton of power available -- more than enough to run the 2X12 very loud, I did that for years with the Focus). Plus, if the H&K goes down, you still have the preamp portion of your "power amp" to use as a backup. (If the power amp goes down, keep a Crate PowerBlock in your car, I still carry one.)
Then take your JBLs out of your cabinet or Twin and build two identical 1X12 open back cabinets. For your small gigs, you take just one of the 1X12s, the new head and power amp, and you have a tiny rig that will blow your ears out if you want it to. Stick the 1X12 on a milk crate and stack the preamp and power amp on top of it. Big gig, add the second 1X12 and you're at full power with only one more thing to carry.
Get one of the newer Holy Grail reverbs and you will be happy enough. Run the reverb in parallel if you get a two channel power amp, it will sound cleaner that way (take one preamp output and plug into channel A of the power amp, take the second preamp output, run it into the reverb at 100 percent wet, then run the reverb into channel B of the power amp and turn it up to taste. (Both the Focus and the Shuttle allowed me to do that.)