Topping DX7 is a dac/amp, correct? If you already have a DAC, then for sure, get this. If you don't have a DAC, note that the Schiit Modi 3 came out 6 weeks ago.
The DV costs 70% more elsewhere except for shortly after a drop, so whenever you want to upgrade to something bigger and better, you'll not lose much on this resale. If you want to toy around with different equipment, buying that equipment in order of loss on resale could be a smart way to go.
I am glad that the 58x begins with a "5," because technically, I don't think it has the chops to hang with the big boys. I think the best technically is clearly the 600, but my favorite to listen to is the 6xx, so technical chops isn't everything.
By this token, I've only begun to appreciate the 58x over the past few days, since the DV was delivered. I'm using it to listen to bassy music as we speak, even with the HD600 within reaching distance. Because of the darkvoice, I'm choosing to listen to the worst over the best. The DV seems to enable the HD 58x to be what it always should have been. I have the Fiio E17 alpen, audioengine D3, and O2+SDAC. None of those do the 58x justice.
In all honestly, the extolled topping is probably superior. It's also expensive. And it isn't a tube amp. Now that I have a tube amp, I know that that means something.
Zeos said that there is a 3 decibel drop in the bass with the darkvoice, but to my ears, it sounds like the exact opposite. I decided to sell one or two of my Senns, and it sounds totally crazy, but I now think the first one to go will be the HD 600. The 58x is better for music with a lot of bass, when that punch and rumble is more important than detail.