The B&W brand is nice, but if I was offered a trade, I would feel no desire to replace my HT3s. The B&W 604's extension only goes into the 40s. My HT3s go well down into the upper 20s. The HT3 is truly wonderful and has the added benefit of being able to be built by just about anyone. The new kits with pre-fab cabinets is an added step of simplicity for those without a tablesaw and router. The front baffle that came with my HT3 was exquisite. (I made my own, mine are all baltic birch ply)
Another thing that is cool here is you can totally customize your speaker with any veneer you wish to use. You can match your furniture easily this way. I went with walnut on mine and I think it looks better than anything B&W offers.
The HT3 will be a great speaker for you, I have heard both and would choose the HT3 any day of the week and twice on Sunday... oh, wait - it is Sunday!