Good review. Similar to how I feel about 717 vs BH. There is one bit, however, which I really want to emphasize:
So my recommendation for anyone thinking about the BHSE is to factor in a really good source to go with it, if you haven't already done so. And by really good source, I really don't mean anything cheap - good sources tend to cost serious money, and really good sources tend to cost even more. Not that something expensive is necessarily good though - but typically good stuff is expensive, that's just how it goes.
This is critical.
You really are at the limits of resolution here, and your source will stand up to the highest scrutiny. Not just that - the BH is a straight wire with gain, and it has no colorations at all, so the tonality of your source will really come out. I hate to say it, but if you don't have a top-quality source or aren't planning on getting one, don't bother with O2/BHSE or even plain old BH. Which is not to say don't bother with O2 period, on lesser amps it's not as revealing, and off a transformer box with a good tube amp it can be quite euphonic, if not technically top-notch.
You will also hear every flaw in every recording laid bare. This phrase is usually reserved for stupidly bright systems that emphasize every bit of sibilance; the O2 is not bright nor does it make sibilance painful, but man, after living with this system for a while I really wish people paid more attention to how they record vocals. It's quite something to hear a warm and rich guitar sound next to a cold, metallic, clinical vocal on the same track. This is how revealing it is; you will hear everything, how each track was recorded, the spacial information for each instrument, the amount of compression on each track, and so on. You will hear flaws in just about everything - and you will have to come to terms that your favorite recordings may not have been produced quite as well as you thought.
When it all works, it's pretty magical. Even with the regular BH the level of transparency is stunning. The O2 pulls off the disappearing trick reasonably well on lesser gear, but it's nothing, and I do mean nothing, compared to this. It's simply total immersion - for better or worse.