Heya,
I will second the FiiO E9 at the $100 mark. Ask anyone with Beyers who also have a Matrix M-Stage and have tried the E9 and see if they noticed a difference. People being honest will say they probably couldn't tell a difference, or that the difference was insignificant, barely noticeable. The FiiO E9 at it's price is ridiculously under priced when compared to similar performing solid state amplifiers which is why it's so popular and highly recommended. But it's simply not regarded as a luxury amplifier because it's a budget amp. It just happens to be a budget amp that is actually very good. You'll find a lot of stuff after a certain price range really just becomes luxury, novelty, etc. The quality increase and your ability to actually benefit it greatly drops off a huge plateau. By the way, getting a $300+ amplifier to power your 880's which are just barely that price range isn't really going to do much. I think you won't be able to tell a difference between a $60 amplifier and a $400 amplifier that are solid state with your Beyers. A tube will change sound enough sometimes for you to notice (and there really are no cheap tubes, so going tube is a dedicated push into the world of tube amps).
Have you considered splitting your budget up? I found that the amplifier for my headphones didn't do as much for my sound compared to getting a good DAC. Good amps should simply sound the same. But they're all amplifying a signal, so if the signal is poor the outcome will be poor. So I hope you're not just outputting from a laptop port or something.
If you're interested in something that will be a great DAC, support anything you want, and be a good amplifier, check out the Matrix Cube DAC. I have one. It powers my DT990 600ohms perfectly fine to high volumes and sounds great as a solid state all in one that has a lot of options and is $270 new, shipped. One of my favorite purchases this year in terms of support my entire collection.
Very best,