Razer Kraken Pro
- Has a more pronounced bass, very heavy and thick
- Bass is uncontrolled
- Bass is rather muddy with no punch, bloated.
- Highs is almost non-existant, very recessed and blanketed
- Mids are fairly decent, surprisingly neutral
- Horrible harmonic distortion, forget listening to piano tracks
- Earpads are comfy... for the first 20 minutes, after that it gets too warm
- Decent mic

Superlux 668b (I assume you're talking about the 668b)
- Bass isn't as pronounced, but is significantly tighter
- Proper, controled punch on the low freq, not a bunch of "wuuuuub wuuub" mud crap like the Kraken Pro
- Mids are detailed with much better depth than the Kraken Pro
- There's a sense of crispness in the highs, but not very harsh that it'll pierce your ears
- Good harmonic distortion, at least listening to most piano or high trebled tracks won't result in white noise
- Stupid headband design, this is a personal complain.

Hope that gives you an idea of the both in comparison.
With everything included? If so, it isn't too bad. The decoder box alone is worth RM200.
Just don't expect the headset to excel at anything at all.

The AX 720 was very uncomfortable for me too, but that was when I hadn't broke the ear pads and headband in. Considering your friend's set is a used one, it should be better.

For me, the AX720 was never comfortable. I found it too stiff, granted I only tested it for 2 weeks so take this with a grain of salt.
As for sound quality, the AX720 as I said doesn't excel in anything.

They're fairly neutral, not heavy on bass nor does it bring out too much details on the highs either. Personally I don't like overly bright cans, but I found the AX720 to be on the lacking side as it is often hard to pick up details.

Soundstage is fairly decent, again not wide given that this is a closed-back headset after all.

The AX720's decoder box on the hand is pretty good for what it is and this is why that RM200 you'll pay for is I'd say, a fairly decent deal. It provides Dolby Headphones for virtual surround, which you can pair it to many other headphones. It's great if you're a console gamer too as you won't need to switch around your stuff.

Good for gaming only though, the box still lacks detail for any sort of music IMO.
If you just LOVE bass, then you'll want to probably check out the Sony XB700. I'm not entirely sure if Sony Malaysia still sells it or not, but the last time I check it goes for about RM400 or so.

The XB700 is so bassy that I think it is overkill, it sounds amazing for bass-heavy music (trance, dubsteb, hiphop, those kinda stuff) and it'll really make your ears numb in most shooter type of games. Unfortunately, it lacks detail in the mids and highs, soundstage is non-existant and don't be surprised you won't even be able to hear foosteps next of an enemy next to you.

Otherwise, if you want a better compromise with some emphasis on the other frequencies... I'd suggest looking at the ATH M50 from Audio Technica. It's immersive enough with plenty of bass, but doesn't overwhelm detail on the other frequencies. Soundstage is fairly narrow, but if you want something truly immersive for RM500+, the M50 is  hard to beat.
Analog over USB any freaking day. I won't even use a USB headset (unless it is just as a mic) if I'd got one for free.

Let's take one example most people love comparing on - Steelseries Siberia V2 and Siberia V2 USB.
While their audio quality largely remains similar, the single one MAJOR difference is the amount of hissing, pops and cracks that is present on the USB version.

That ALONE pretty much breaks the deal for me irregardless of what other features it brings.