Bi-wiring does not electrically separate the two halves of a crossover.
They are connected together by the two pairs of speaker cable, because those cables are connected to each other. The electrical properties of any decent speaker cable are so vanishingly small that it makes a vanishingly small difference regardless of whether you are using 0.8, 8, 80 or 800cm of cable to achieve this.
Any difference is because you have just replaced the electrically distasterous jumpers with a competantly designed conductor, regarless of it's length within reason.
That different constructions and materials can make small differences is not in doubt, but even these effects are often overstated. However, the act of biwiring involves removing one poorly designed conductor and replacing it with another of entirely different construction. It is this that makes the difference, not any bogus claims about 'separating' or isolating the crossover halves, because they haven't been.
The only way to achieve that, is to connect the two drivers to separate amplifiers and that does make a difference.
I would only like to say that the better the sub, the less it sounds. If a sub has a definitive tonal character, then it really has fallen at the first hurdle. Anything that can be defined as tonal, really should be coming from the speakers.
Like me with my MJ Ref 200 before you, you're onto a win-win if you buy the BK. It was a leap of faith, but I bought my Monolith for less than the proceeds gained from the sale of the MJ and I have never looked back. There was the steep learning curve of integrating something with real power and low distortion, not to mention the initial "where did my bass go?" common to all owners with their first low distortion sub.
I didn't realise you knew all there was to know. Good for you!
'High end' ownership is no protection from the accepted norm. You're clearly happy accepting what you read in the printed press, so I'll leave you to it, although I'm still wondering how, inspite of all this knowledge, you ended up with an 'R' Series REL?
They're mid-fi priced with low-fi performance, but you can call them hifi if you want.
When you've taken the time to hear real high end subs (as defined by performance rather than price) from the likes of Velodyne, SVS and BK back to back in the same room against the mainstream favourites, report back and see if your preconceptions have changed from from what you've been told to expect.
Until then, I'm sure you'll be happy with REL, B&W or MJ Acoustics at whatever prices they choose to charge. Your loss.
The subwoofing community is also fortunate that there are a a pair of particularly commited individuals, one Engliish, one Finnish, who have undertaken extensive and completely independent testing of a number of products. You can find their tests here and here. Read the well written FAQs to get a grip on the more complex measurements. It took me ages to get my head around some of it.
You'll note that these tests do not take prisoners - poor performers stick out like a sore thumb with curtailed and non linear responses, high power compression, high proportion of high order harmonic distortion, poor impulse response etc, etc. However, it's particularly worth trying to understand 'Group Delay'. All so called 'musical' subs do this particular measurement well. Even if they screw up everything else, they will get this right. This quality is not mutually exclusive though, it is perfectly possible to combine it with solid performance elsewhere.
In the light of this, compare the REL R-305 with the XLS-200 at half the price. Pay particular attention to the scarcely believable distortion figures and note the enormous amount of clearly audible high order components exibited by the REL. Interestingly, this would result in a similar quality of bass to that produced by some fairly serious floorstanders ie, not actually very good, but it would sound familiar so people think it is good.
It's not. It's inaccurate and wrong.
It's worth reading the British test too. The big RELs are good subs (although expensive) and the T-1 shows that REL have stopped peddling the rather poor Q Series and are back to trying to advance the art with innovative products.
I'm afraid that you've repeated any number of common cliches and disinformation, oft found in the printed press and without a basis in reality.
You DO want a subwoofer capable of high SPL, because if you then run it at more moderate volumes, you benefit from the lower distortion of a device running well within itself. It's completely illogical to buy a sub woofer that just goes loud enough, or just goes deep enough - You wouldn't buy an amp or speakers that only go as loud as you listen, so why is a sub different?
As regards integration, the sooner people realise it's that the room has by far and away the biggest bearing on subwoofer performance, the sooner they'll stop buying weak subs in an effort to avoid the hard work that really good integration entails.
The wavelengths of sub-bass are in the same region as the dimensions of a room. As such, huge peaks and dips in the frequency response can result from standing waves and cancellations within that room. All to often a sub is descibed as 'boomy' when in fact all that has happened is that the sub has proven capable of reaching down into the room mode frequencies. Traditional hifi 'wisdom' is to replace the superior sub with a fartbox like a PV-1 because it 'integrates' better. In fact what has happened is that the sub user is sidestepping the issue of correct placement and tuning and in the process loosing a flat powerful, low distortion response.
As is self evident from this potential dominance of the sound by the room modes, when you demo a sub, you are intrinsically demoing the room in which it is in. Unless the dealer in question has a room the exact size, of the same physical construction and exact same layout as your own, then you haven't really done much more than work out what sounds good in his room. The only sub demo worth a pig-in-a-poke, is in your room.
The thing is, subwoofers aren't terribly complex beasts; nothing like as complex as a speaker for instance. They operate at frequencies too low to have a sound of their own and they are extremely measurable because even the smallest sub measurement takes significant preportions of a second. I know it offends the hifi ear to suggest that you can measure the absolute performance of a piece of hifi, but you can. There is a curious correlation between the manufacturers that say you can't and how badly their gear measures though. Coincidence?