Fast Track C400 generates too much noise at instrument input, noisy amp simulation

[TLap]

Hey. I've bought my Fast Track C400 unit at about early 2013, if I remember correctly. I have bought it in order to record my guitar using digital amp simulators, and also to be able to record using a microphone.

I've never really recorded anything with it because the noise levels I get while attempting to use guitar distortion effects make it unusable. I've so far used it for practicing with the AmpliTube 3.5 software. The vast majority of distortion effects ranged from "somewhat noisy" to "HOLY COW, IT'S LIKE I'M RECORDING AN INDUSTRIAL SHREDDER". I've had a couple of people suggesting me to use a different amp simulator as they didn't think highly of AmpliTube, but I knew that AmpliTube is not all to blame as the signal it receives is characterized by it's SNR, and mine's apparently not good.

I have eventually did get another software - Peavey's ReValver 4, and while I do like the authenticity of its sounds and its general handling of noise much better than AmpliTube, the noise is obviously still there whenever I use heavy distortion or other effects to which a quiet input is crucial.

I have suspected it could be a grounding issue, so I tried running everything on a battery-powered laptop. It didn't help.
A feature of the issue that could probably shed light on the matter is the fact that the noise (when the distortion effects are applied) is audible even when there's nothing connected to the interface; connecting the guitar changes its tone from a hiss to a hum and amplifies it some more, but it's definitely there in all of its glory even when nothing is connected. Raising the 'Gain' knob increases the hiss accordingly.

I'd also mention that the instruments' preamp is surprisingly easy to clip as I usually play with the 'Gain' all the way down (no 'Pad') and still a good strum on my Les Paul Studio gets it to the reds. Using the pad obviously helps lowering its levels (and the FT's own internal hiss accordingly, but only up to a point), but frankly you don't want to use an incredibly low gain on the preamp of a guitar for heavy metal recordings - its tone gets too brittle (and it still doesn't solve the noise). If I'm already discussing the gain levels, I would also mention that apparently the knob doesn't grade it correctly all the way: you're probably not expected to use the interface with such settings, but somewhere near the upper end of the gain knob there's a sudden jump in the noise levels at which the 'Pad' button doesn't lower the noise anymore - it even raises it a bit!

Anyway, I hope you have an idea of what's the issue and how can I fix it. If it's a known issue then it's infuriating as it renders the interface almost useless for me. And if my own unit is suspected to be defective - is there a way to fix it? I'm from Israel and I have a feeling I now won't be able to receive a replacement here (though it has performed this way pretty much ever since I've bought it).

Thanks in advance.