I must apologize for not having been clearer in my post. I'll try to sort out the confusion.

When I talked about bookshelves, Persian Carpets, and 'the mirror technique', I was referring only to the mid and high frequencies.  I had already discussed dealing with bass frequencies by 1. positioning, 2. bass traps and 3. equalization.

Of course a Persian rug would be virtually unabsorptive at anything much below mid frequencies (in fact, it wouldn't be much use with the lower mid frequencies either).  Mounting the rug on a sheet of polyethylene (or similar) 1" or 2" thick, can reduce the frequency at which it's effective.

If the room is acoustically hard, it's often because it has a lot of glass and/or concrete. Bookshelves are considerably less reflective than the concrete walls that they are mounted on and I have found them to be highly effective in mitigating the effects of an acoustically hard room.

Again sorry for the confusion. This should be taken in the context of the rest of my post.

Of course I didn't mean that the bookshelves, paintings or rugs were as effective absorbers as anything else. I said this in the context of my views on the brain automatically compensating for moderate acoustic differences between rooms (except in the bass frequencies). This compensation means that full acoustic treatment for the mid and high frequencies is undesirable in my opinion as it gives an unnatural feel to the room.

In this context, bookshelves and paintings are as effective as anything else, for improving the acoustics of a hard room, in my view, while Persian carpets are ideal.

I hope that this clarifies my comments.

The Nonsuch 4s are designed with a small vertical array drive unit. This is over 20cm high but only 5cm wide.  Thus, while horizontal dispersion is similar to many conventional loudspeakers, off-axis vertical dispersion is reduced.  There is a considerable vertical sweet spot which makes these 'sit down' speakers. You have to listen on axis to hear the full frequency response. Stand up and you'll lose the high frequencies, while at floor or ceiling level the mid and high frequencies are considerably reduced relative to the lower frequencies.

One benefit of this approach is that the negative effects of the early floor and ceiling reflections are minimised.