So the summary so far is that if you don't want any play or stability issues your choices with a heavier camera are:
A/S Cube
A/S D4
Linhof 3D Micro
KPS T5
PhotoClam Multiflex
The KPS T5 is the most economical, but all are expensive.
I know a cheap head like the Manfrotto 410 works as I've used it myself, and I've seen others use it that could easily afford a more expensive head. It's unique as it has all three axis geared which is nice in some situations, and I think it's the fastest to operate and most user-friendly head. I think it's the great operational ergonomics which makes some keep the head even if the quality is so-so, the controls are indeed better than my D4. However for me the losing-position-slightly when I slided the sliding back got worse and worse with time (tried to tighten the head but did not succeed), and the 1400g+extra for (the excellent) Hejnar Photo clamp extension felt heavy to carry in the field.
Actually I think the weight was the stronger reason for me to upgrade than the play. I've haven't really got rid of the need of after-adjusting after I slide in the back, as the tripod footing is often soft soil and some shifting in the tripod itself often happens anyway.
I looked to the Cube but the extra weight and not being able to unlock made me go for the D4. Some see the inability to unlock as a safety advantage though, so it's personal.
I actually did hurt my finger the first day using the D4 due to accidental unlocking, but that was because I'd never used a ball head and was used to the 410 head so I thought you didn't need to apply force on the locking knobs to lock it. *Tilt* *wham* over my finger.... you learn effectively through pain though, I've never done that mistake again
The D4 is precise, but as the rotational axis is inside the head it's not really "silky smooth" to operate with a heavy camera on top. You need to use some force when turning the knobs as you move quite some mass, so it's a bit jerky, but there's no play and fine-tuning is no problem. I assume the Cube has a smoother feel as you shift less mass due to the rotational axis being closer to the center of mass of the camera. The couple of hundred grams lighter and ability to unlock is worth the jerkiness though.