just a question, what amp are you currently using for your N805 speakers?
I have only heard the 805-signature, and the new 805S models. If these are any guide they are extremely revealing speakers and deserving of a very good amp.
Of the three budgets you mention it is probably only the higher end that will do these speakers justice.
I would imagine that the N805 needs a high quality solid state integrated some thing like a Mcintosh, Krell, Lavardin, Classe etc etc
But you would need to listen for yourself to make a decision.
For good bass response, you may want to try one of the Mcintosh integrated amps.
Otherwise, you could consider a sub, but I personally dont like subs.
I see from another thread that you own a NAD BEE combination. So I guess that answers my question, also you state that you listened to some other amps (Arcam, Krell, and Musical Fidelity).
I am envious of your high resolution speakers, but see that this is a little unbalanced with your present electronics. Hence your dilemma.
I have listened to the 805-signature model, and the new 805s, the dealer had Electrocompaniet amplification in use at the time I was listening.
I think that if you upgrade to a more powerful and refined amp, you will get tighter and faster, more damped bass from you N805. So you will get more quality bass, but not a great increase in quantity.
The two 805's that I listened to, I would say their strengths were their neutrality, lack of boxy colourations, imaging, and speed. But I would not say bass extension is one of their strengths. I find that Dynaudios of a similar cabinet volume offer greater bass extension.
You may need to get a powered sub, but I don't know anything about subs, so cannot advise. But I am sure B&W probably make a complementary product within their range.