It's sort of ironic that Toots' name is on this one. There is the Toots Hard Bopper and the Toots Mellow Tone. You can often find the Hard Bopper in stores, but I can never find the Mellow Tone. Yet Toots plays the Mellow Tone (he really does - the first time I ever saw one was in his hand, and when I've checked out his gear at gigs and record dates, his harps are always Mellow Tones and 270's. Come to think of it the *only* times I've seen a Mellow Tone, it's been in the presence of Mr. TT.).
Toots like to play very softly, and the Mellow Tone is characterized as being for "ballad playing." The Hard Bopper, on the other hand, is built for loud playing, something Toots never does (he's asthmatic, doesn't have the wind, built his style and his chops around breath conservation).
So does the Hard Bopper live up to the hype?
It has thicker reedplates than a 270, and they're chrome or nickel plated, I forget which. This makes their surface harder, and the greater thickness gives the reeds more swing, which means more loudness (and greater dynamic range). It also means greater reed fatigue. The reeds are *supposed* to be copper molybdenum for longer life, but my experience, and that of some other players I've talked with, is that the Hard Bopper reeds wore out way too fast for such an expensive and supposedly high quality instrument.
William Galison attributes this problems to one of Hohner's shoddy periods - they seem to go through cycles of bad quality on particular things - around 1988, for instance, I had a lot of trouble with slide springs breaking. In fact William characterized the reed problem as something of a mini-scandal. This was in a discussion we had nearly two years ago, and seemed to be in the past then, so you may not have a problem. It will definitely be louder than the CX-12, and to some extent louder than the 270.
As far as a "vicious attack," I'd say you might want to look to your technique and adapt it so that you can get a vicious *sounding* attack without actually being vicious. Respecting the limitations of an instrument will generally get you farther than ignoring those limits.
Which is not to say you shouldn't try to push beyond them. But often brute force won't work (sometimes it will :->). Finesse in approach, distinguishing the means from the end, and working on the instrument itself will yield better results.
The one thing you may be able to do to the instrument itself if it's choking out on a hard attack is to raise the offset of the reeds in question - widen the gap between the tip of the reed and the reedplate. Raise it too much and it won't respond to soft playing, but you may be able to find a happy medium.
You might also want to work on the size of your oral cavity - the resonant chamber - when playing the bottom octave. It may be too small, thereby militating against an easy activation of the reeds.
Also work on playing louder without increasing the breath volume. Work on your sharp attacks starting at a very low level, then gradually increase the intensity so that you can attack it more abruptly at greater loudness. When you max it out, back off a little, and work on refining your control at that level for awhile before going on.
Do a little each day, and keep it up. This stuff doesn't come from "cramming" overnight.