The Energy sub is more of a bass module than it is a subwoofer. It has an 8-inch driver with a 60 watt RMS amp (240 watts peak). It is probably a very nice unit to have in a small den or bedroom. It certainly does not belong in your cavernous room (poor thing has been abused). As Mix_Master indicated, he has owned both the 15-inch model and several 12-inch Dayton models, and feels the 15-inch model was a bit sloppy for his taste and actually preferred the 12-inch models. The Dayton SUB-1200 has gotten pretty good reviews (I haven't seen much on the 1500) and will get down to around 25 Hz (where your Energy will get down to perhaps 36 Hz -- not great, but not bad for its size). But plenty of budget subs will get down to around 25 Hz. A budget sub far too often equals budget sound.
As for your question about being a modest upgrade, I'd say it's more than just a modest upgrade. But the fact is, your room is huge and cries out for anything BUT another budget sub (kinda like throwing good money at an impossible situation and hoping for the best). I will restate my advice. Keep the Energy for now. When funds become available go for a subwoofer from Hsu, Outlaw, PSA, Reaction Audio, Rythmik Audio or SVS. These I.D. company subs start at around $500 for Hsu, Outlaw (when on sale), Reaction Audio and SVS. I'd wait until you can come up with $800 and you'll have a sub that will have a chance of giving you what you seek (as long as you have an eye on a second sub in the future). If that's not in the cards ever, then look for the best $500 sub you can buy (SVS PB-1000, Reaction Audio BPS-212 or Hsu VTF-1 Mk2).
I don't know what else I can say, except I wish you the best of luck on your choice.