man oh man, i feel your pain. i ditched the rm100 for that one reason...weight! i'll spare you the long path to my current rig but i've ended up with the ultimate rig IMHO.
RM4 + mesa 20/20 in a single 5U rack (along with Xpression and Furman).
total weight about 50lbs. the skb 5U rack is also a roller rack. stack my footpedal on the rack, carry guitar in other hand = one trip. 2x12 NXT cab = 1 trip. total trips to car: 2.
I loved the RT2/50 but did not cure the weight problem. Went thru several other power amps and I cannot say enough good things about the mesa. It weights 14lbs and packs the punch of a 50/50.
do the "deep mod" on the mesa tho; takes about 15 minutes; details available online.
I went the rack route for maximum versatility... not weight.
My rack is 38 kilograms, 84 lbs!!
Every rackunit has a tough casing, where as a head has it all in 1 shell.
So don't mistake between that.
Also the RM100 for instance has 1 power transformer vs. 1 in the RM4 and 1 in the RT2/50.
On the other hand an EL84 poweramp will defintly be lighter.
for a rack casing I'd say go for the SKB X-rack, very tough, easy to carry and light(ish). Plus no rattling handles.
Also it you turn it upside down and mount everything that way it has 2 perfect feet for standing on a cab