Originally Posted by Anarion
IMO, Forte's headphone amp is unusable with HD 595 and HD 598. Tested with both, it's just too loud without some kind of external volume controller. Also it doesn't work like that, they both output sound at the same time. There's no switch. Auto mute thingy only works for front panel connector and I wouldn't use it, much more likely to get inerference. Zx has handy switch in driver though.
Never had any Sennheisers to test, just an AD700 (which can be driven by anything) and my Stax Lambda setup (which needs external amplification anyway, like all electrostatics). If nothing else, it was LOUD, to the point where the usable volume range was more like 5% (anything less is mute) to 7% without having my ears blown out. (Volume isn't necessarily an indicator of amplification quality, though, and I just didn't have the right headphones to test for that.)
I distinctly recall the X-Fi Forte having a checkbox to either let both speaker front left/right out of the breakout dongle and the dedicated headphone-out both play at the same time, or mute the speaker line-out if the headphone-out has something plugged into it. Maybe I should've tested it a bit more at the time.
Unfortunately, as you say, front-panel audio jacks are generally useless because they have ground loop issues in most computer cases, among other things...