@hellzington has had both and kept the Grace. He can tell you about it. I preferred the Grace too. The Empire had a lot of overlap with Diezels and Rectifiers, but is only one channel. Unfortunately I think most of his amps are too expensive for what you get feature-wise.
Yeah, I owned an Empire and still have my Grace. I greatly prefer the Grace. Like
@fearhk213 said, the Grace is a chameleon of an amp that has
everything to do with your choice of guitar and that guitar's volume knob. It's extremely responsive to different pickups, volume/tone settings, and different guitars altogether. I am 99.9% sure it's based on a Trainwreck Express with some SLO DNA. (See more in the video below.) It's just a ton of fun to play and I enjoy it every time I plug in.
The Empire is something different. It has a thicker, juicer midrange and is a bit darker. It didn't feel as punchy as the Grace and doesn't respond the same way to your guitar's volume knob. It "cleans up" like a traditional amp instead of "morphing" like the Grace. Overall a much smoother, darker, more polished sound. As far as features, I remember the sweep of the gain knob is a bit better than the Grace. The boost is nice but I felt like it sounded better with it on, so I'd view it more of a "gain cut" than a "gain boost." The Response knob on the Empire is much more impactful than on the Grace. The Edge control is interesting... Brian told me it is not a presence control; it's a high pass filter. Zach demos it well here. It's intended to restore brightness to the amp if you dial the Response up.
I ended up selling the Empire and keeping the Grace because the Grace was far more versatile and, I think, far more interesting of an amp. As
@Yehuda pointed out, the Empire kind of only does one thing. If that's your thing, then cool; you'll love it. But it doesn't really do any cleans at all and I felt like the lower gain tones weren't as inspiring as the Grace. The Grace is simply a
beautiful sounding amplifier. But then again, I really like that spitty, fizzy thing that Trainwrecks do. The Grace is like a more modern Trainwreck Express on steroids. So it's a fit for me. The Empire was cool, but I just felt like it was a bulldozer and I could get that kind of tone from my IIC+, Blueface VH4, or Naked; and all those amps are more versatile.