Anything that is FRFR (full range flat response). I've been using a powered Mackie monitor for both practice and stage monitoring. Seems to work pretty well.

Does the cube 30 have an FX loop?  The problem you are going to run into in trying to use the patches that you like in the headphones through an amp is that you have to change modes and also make adjustments to your patches.  You will most likely have to do the same even if you get a DT25.  IF you follow the advice of samuelito2011 and/or tdollaway you may be able use the same settings as you use with your headphones. 

Through the headphones you want to be in studio/direct output mode.  You will also likely (but not always) using full amp models as opposed to the pre models and speaker cabinet models.  This is also how you would run through an FRFR system (which would include a keyboard amp).

If you want to run through a guitar amp, you will need to change the output mode to combo/stack front or combo/stack power amp.  The prefered method would be combo/stack power amp if you have an FX loop on your amp and you can simply plug into the FRX return.  Then, in a lot of cases you will possibly have to copy all your settings from the full amp version and switch to the pre amp version of the same amp model (but not always, it sometimes sounds fine on the full version).  And in a lot of cases you will have to disable the speaker cabinet model.  IF you don't do these things you will essentially be running a modelled power amp through an actual power amp and a guitar cab emulation through an actual guitar cab. This doubling up especially through to stages of the tone shaping is going to colour the sound to the point where it doesn't sound much like it did in the headphones.  The same holds true with a DTxx series amp the only difference is you can connect using a L6 Link and it will automatically change some settings for you, but you are going to want to run pre models of the amps and disable the cabinets in most if not all cases.

There are alot of great FRFR options in the price range of the DT series amps (and cheaper). For me personally I don't think it just "seems to work pretty well" (tdollaway was probably understating the success of this setup) I think it sounds better than playing through an actual guitar amp and I have an exponentially easier time dialling in tones than I did when I was using the HD500 with an actual guitar amp. I would never want to disuade anyone from buying a DT series amp, I love the concept and even on their own without an HD500 they sound great.  I just think that before you lay down a bunch of cash you should flush out all your options and make sure you are getting something that is going to work for YOU.