Any bass amp, keyboard amp, or powered speaker should work fine for you. I currently use an old Crate KX-80, it's does a good job except it's about 40 lbs too heavy for my taste. My next "amp" will probably be a Behringer B215D powered speaker. I like amps with 15" speakers because I play both boogie-woogie piano and heavy low synth tones, and I want good frequency response down to ~30Hz.
The B215D can be sat up like a normal keyboard amp, or turned on its side like a wedge. It can take balanced or unbalanced inputs, and can be chained. I believe if you run a unbalanced line in and an unbalanced out to your FOH board that you don't need a DI. Oh, my Crate also has a balanaced out, TRS though, so I keep a TRS->XLR cable in my gig bag so that I can give the sound man a balanced out without worrying about a DI. They generally appreciate this.
You can also run it as a generic monitor if your FOH board can give you a separate mix. Rated at 550 marketing watts, they make about 300W RMS, which is more than you'll ever need if you care about cost. You can also pole-mount them and use them as FOH speakers if you care to. ISTR the output SPL can hit 126db at 1m.