Imp. Stout not bitter enough... options?

So I've got this Imperial Stout I brewed up back in early October.

It tasted okay for being so young, but what little I tasted I never caught on to. I aged it on some coffee beans and a few cocoa nibs as well.

Tasted it again and thought it was just too damn thin, and kinda hot to be honest. It's clocked in around 11.5%. So I decided to add some lactose to it after the fact. Boiled up some water, added the lactose and boom... Beer has a great mouthfeel, the heat is pretty much going away at this point..

I kegged it and let it sit for a while. Put it on tap a few months back and tasted it.. it's a great beer... but it's not bitter enough. It's got a smooth roasty note, but the acrid bitterness I wanted, isn't there!

So I've got this 5 gallon keg of this overly sweet stout sitting here taking up room. I don't have the heart to dump it, because I have hope I can add something later to make it more bitter.

Any options or thoughts?

I thought about getting some water, and boiling up some magnum in there, and trying to blend it perhaps? Dump maybe 1 g of the keg out and brew a dark something with alot of roasty malts, and massive bitterness, and then blend it after it ferments out? I'm at a loss.. if I have to dump I will, as I need the keg, but I really hate to do it. Beer has some potential, other than being sweet.