Evening all. I have comment to pass if you're interested.
Bass directionality is not as closed-book as many suspect. Basic physics, including Huygens' principle, shows quite clearly that bass frequencies emitted from a dipole source will be less directional than high frequencies. The localisation mechanisms (principally interaural phase difference IPD, interaural time difference ITD and outer ear diffraction effects) are far better geared up to providing better directional discernibility at high freqs. So yes, you can without question tell the direction of HF more easily and with greater acuity than LF. That's very different to the question of whether the room is excited differently according to the direction of the sub. More on that below.
Most would agree that the optimum position for a sub in a small room is the point at which there is minimal undesirable excitation of the room (i.e. nasty modes) and maximum desirable excitation (the modal balance found by-ear or that expressed on the room curve of choice). The most significant aspect of room mode excitation is the oscillation of walls but almost-equally significant, and ignored 99% of the time, are the excitations of the flanking paths of the room (i.e. joists, framing, superstructure, window frames, etc). The fundamental frequency of the parallel pairs of walls in a simple room are dead easy to find with an accelerometer. Oscillating flanking paths on the other hand are far more complex mechanical systems so finding out what they're doing is much harder. It can be done - I use a combination of Insul (insulation performance prediction), Odeon (room modelling/auralisation) and my own funny brand of longhand maths to figure it out, but it's not for the layman.
What you can say is that the notion many enthusiasts have about positioning a sub according to a model of wall/ceiling/floor modes really isn't any good and is why you can very often greatly improve a "professionally-tuned" room using lots of trial and error. In fact, sometimes you can just do it instantly, which annoys the person who did it originally!
It *is* subjective, there is no perfect listening room and I would say therefore that the only way to tune a small listening room is to use lots of trial and error. It's a less vague science with large room design, funnily enough.
Sub directionality: very simple actually. If you excite the room structure (so effectively the flanking paths) strongly at a weak point, the room will be excited awkwardly. If it is excited at a stiff junction, i.e. a corner, the room vibration will be more uniform. Think of a rubber climbing frame (go on, you can do it. I know it's bizarre but try) - if you grab one of the bars in the middle and shake the whole thing, it will wobble like jelly. But if you grab a corner and shake it, it will seem more solid. Erm, does that little visualisation actually help? Hope so. Now looking at the polar pattern of a sub, there doesn't seem to be much difference as you move around the cab, but there is always some because the cab is not a point source. Therefore, pointing it in any one direction means that there will be an increased or reduced excitation at the adjacent section of wall/ceiling/floor, so you're treading the fine line between flanking path and wall excitation. So YES, it does matter which direction your sub is facing in and NO, there is no easy way to figure it out. I like that last part because it keeps me in work.
Bottom line, the most sensible way of tuning a small room, assuming that you aren't constrained by time, is to try lots and lots of different positions, rotating the sub (and putting it on its side or upside down..... honestly) until you find a sound that you really like. Jaycan is probably pretty close in saying that the corners of the room are the best places for subs in the context of either stereo or one of the home cinema flavours, although I would always try a bunch of strange positions because you might end up exciting a set of joists that emphasise the room character. Experimentation all the way!
Remember the stale ears rule though - 15 mins of tuning, 30 mins of something else.
