Very quick impression on the Mixamp 5.8 vs the Victor SU-DH1 with Mass Effect 3.
The Victor SU-DH1 is fairly weak at driving my HE-400 alone. It's slightly below moderate volume. With the E17 hooked up, I need the E17 at 48 volume (+12 gain), where with the Mixamp 5.8, I need it at 38 (+12 gain). I'm able to use the Mixamp 5.8's optical out (bypass), so I can just hook up an optical cable there, and feed it to the Vistor SU-DH1 (or Creative Recond 3D which I haven't tested yet). No difference with just feeding the optical straight from the PS3 to the SU-DH1. I couldn't detect any hiss off the SU-DH1, though the HE-400 has a very quiet noise floor even with the 5.8. I wish I had my KSC35 to test hiss out. I believe the Victor is quieter, but slightly thinner sounding than the Mixamp 5.8 (probably a weaker amp).
So quick A/B comparisons...
The Victor SU-DH1 in DH2 mode (Dolby Headphone 2, which is the same processing as the Mixamp/DSS/AX720 use), sounds pretty identical to the Mixamp 5.8. However, the Mixamp 5.8's soundstage projects a LITTLE farther out. Very minute difference. I'd say it's probably the internal amp differences.
Impressions on DH1 and DH3.
DH1: Definitely smaller virtual room size. The positioning was identical to DH2, but the smaller room size I felt hurt the rear positioning a bit. DH1 actually sounds VERY good, and less processed than the typical DH2 that I'm used to listening to. It wasn't a major difference from DH2 other than soundstage size. Those coming from straight stereo would probably enjoy DH1, as it doesn't alter sound fidelity as much. I actually rather like this mode, though overall, DH2 is the best of both worlds, which is why it's the popular choice for gaming amp makers.
DH3: Okay... yeah. The soundstage opened up a bit, and really allowed rear positioning to come up better. HOWEVER, it has a very processed and artificial sound. I wouldn't trade off the more natural sound of DH2 for added soundstage with a echo-y, tunnel sound. I'd say that for very hardcore/analytical gaming where pinpointing is more important than raw SQ, DH3 is actually better than DH1 or DH2. This is a mode that I feel would be for the AD700 owners. I know some people care about DISTANCE of sounds relative to you, but TBH, I don't think headphones do distance all THAT well. I focus more on directionality than distance.
Okay, While I still have the game hooked up in Dolby, I may as well attach the Creative Recond3D and compare the processing to the Mixamp 5.8.
Afterwards, I'm changing the game to output in DTS, to see how the Victor SU-DH1 fares (though I can't do a quick A/B with DH or THX Surround, as those work with Dolby only, and you can't change from DTS to Dolby on the fly).
Quick minute update:
The Recon 3D is TOO weak for the HE-400. It's even weaker than the Victor and the Mixamp. So quick positional cues test without the E17: Both THX and Scout mode were... lacking. I hope it gets better with the E17 attached
Update 2: Scout Mode is considerably louder than THX mode. However, I felt scout mode had WORSE front/back depth, and sounded more like stereo. THX mode is KIND of like Dolby Headphone 2, with very good front depth, and side width. Rear placement was slightly weaker and lower in volume than Dolby Headphone.
Update 3: I really like the sound quality of the Recond 3D USB. It has a fuller sound, and sounds less processed than DH2. However, the front to side positional cues pan a bit harder and faster than Dolby Headphone which sounds more natural in progression. Rear placement is definitely not as easily discerned as it is with DH.
These are VERY early impressions, so they are subject to change.