According to Jason, that's the distortion of the amp with the wallwart, so with a linear power supply, that could be quite an improvement
The wallwart is a "16VAC transformer", so internally there is filtering and rectification down to +/- 15VDC. It's most likely a linear power supply as is. There's little reason not to make it that way, and a quick look at the board doesn't suggest to me otherwise (though I haven't looked at nearly as many amp internals and bare electronics as many others). As for the benefit of even better power supply filtering, that depends on the design. It's often not a limiting factor, but of course in some audiophile gear designed to do things the hard way, it can be. And usually issues are manifested in noise levels, not as much distortion, as far as I know. If the design has high PSRR, a suitable switched-mode power supply or whatever else may not be any real issue.
Effect of load impedance and output power on distortion figures is going to be a lot greater. For example, if you're citing results at 1V into 600 ohms, who knows what it could be at 4V into 32 ohms.
The modi is; "USB Powered, No Drivers Modi plugs into virtually any computer—Windows or Mac—and requires no drivers to deliver great sound, up to 24/96 sampling rates. It features asynchronous transfer with individual crystal oscillators for the 44.1 and 48k sampling rates, and uses the USB Audio 1.0 standard over USB 2.0, and is powered by the USB port."
Sub optimal in my opinion. PC power is flaky and dirty and better results are usually obtained by a dac that has it's own power supply. If you use this I would at least use an external powered USB hub instead of plugging it straight into a computer. That makes a night and day difference for my HRT MSII.
Again, depends on the design, what they're doing. Obviously they're filtering USB power, hopefully as much as needed, and optimized well. Suboptimal can still be very good (not that I'd assume necessarily off the bat that they did a most excellent superb job, particularly at that price they're charging). DacPort and ODAC do pretty well just off of USB power, for example. Anyway, as you say, you can use a powered hub if you want.
Out of curiosity (sorry, I just have to ask), did you ever run some benchmarks or more formal tests comparing your HRT MSII with the hub and not?
No offense to you or what you say in particular, but USB hubs and power tends to be an area of audio tweakery that IMHO just intuitively makes sense and obviously could make some measurable impact, but the effects ascribed to it often run away into the deep end. Like many other things in audio, I suppose. I'm not saying that weird effects can't happen, but I wouldn't leap to certain conclusions off the bat without more satisfying data.
That said, if you're getting something of ground loop issues connecting to an amp powered from the wall, I guess it could be readily different using a hub or otherwise getting ground from somewhere else.
