I haven't heard the HFI-650 but it's looking almost like a repeat of the HD280 - I bought them because a 'core' of people here said 'the bass is great' - but I found that they didn't have any to speak of, not for my musical tastes at least. Now, you'll find a bigger core saying the 280's are recessed in bass - and this is only after more people who actually have the things have posted their views... Get the picture?
If you want to find out what an HFI-650 actually sounds like, I think you'll have to get one and see (and please let us know what you think), or have a detailed PM chat (if he's willing) with kelly who seems to be the only regular regularly proclaiming his ownership. I argued headbands with him so I'm pretty certain he actually has one.
The 280 doesn't actually lack response. It does actually go very low, it's just that the proportion of bass volume is a little unbalanced to what seems many people's ears. Perhaps if you have (audiophiles, cover your eyes now), EQ?
Also, referring to a post earlier, "The 280's are like the beyer 250-80's but with less impactfull bass and less recessed highs..". The Beyers seem to be increasingly compared (by the 250-80 fanboys) with headphones that they are nothing like, often in the 250's favour. Beware of bullsh!t. The 250's are very undemanding phones but there's really not that much to get excited about, especially at Headroom's price relative to many other phones. Personally I was more buzzed with the 280, even though I didn't like them at first (and to be honest, not that much even now - but they're prefect for my current use, monitoring)
Have you considered some of the cheaper closed Audio-Technica phones? I've put a curb on the acquisition of new phones for the moment, but the new crop of ATH-A500, 700 and 900 seem very interesting. I like the response of the ATH-A9X, but have no idea whether these will be similar. If you wait around, some 'early adopter' will hopefully post their review...