I have a Boss Tube Amp Expander and i love it. It's much more than an attenuator: you have eq, reverb, delay (Boss top of the line effects), you can simulate cabs and different mic setups. You can use the USB output to directly record on PC.
The quality is superb, the sound amazing. I especially use it with cranked amps to have the best distortion. You find amp's sweet spot and then adjust the volume to the situation you're in, so you have the best tone you can desire at the right volume.
The Boss is not cheap, but in my opinion is worth every cent.
As the Tube Amp Expander is scalable from 100W to 50W to 10W, the great thing is that you can take a small 5-10 Watt valve amp, crank it up and make it a 100W amp...this really make a glorious sound (in this case is not an attenuator but a real expander). The Tube amp takes the input signal, replicate it (digitally) and then you can output the sound at whatever level you want. Technically you are not in real tube amp territory anymore, but i can assure you that the system works so well that you can't tell the difference between the amp alone and the Boss replication.
Someone talked about the UAD OX. I've found it limited in the attenuation settings, as it's not linear in the volume attenuation: you have 4-5 steps and you can hear huge difference between them. The Boss is so linear in the volume reduction that is like a Hi-Fi volume control, so you can really find the perfect spot for the room or the situation you're playing. And even at the lowest volume the tone is 100% there, you don't lose a thing.
The app control is good and works great, but in this case the OX seems better, or at least more appealing.
In my experience this attenuator changed the way i can now play guitar. I would have never reached such glorious tones in a small room situation or at practice levels without it. Now i can really have the best tone from my tube amps whenever i pick up my guitar...heaven on earth
On youtube you can check up some great reviews from Tom Quayle and Rebea Massad