True. He used DEAD strings in 2004 and still got mighty power. The reason: his stroke was the conventional across the body finish and much flatter in general. In fact, he regularly hit 100 mph+ forehands on rally shots, and he never does that now - only once in a blue moon. It's funny to see a baby Nadal cranking up BIG forehands, bigger than he does now. His forehand also used to penetrate the court much better then. If he had kept on with it, he probably would have had more slams, done much less running and wouldn't have been injured. Not to mention more people would like his playing style.
For whatever reason, starting late 2005-early 2006, Toni made Nadal switch to a exaggerated reverse finish close to his head. This enabled him to get more spin, but at the expense of pace and penetration. Without doubt this was a great tactic against Federer, but it hurt Nadal's HC game, especially against the rest of the field. I recall Nadal getting blown of the court by big hitters before 2008, and I used to think that the 2004 Nadal wouldn't have lost to these guys.
Whereas in 2004 Nadal TRULY an aggressive player (who relied on his forehand to hit winners at every opportunity) from 2006 Nadal became a counter-puncher whose first instinct was to defend. It was not until 2008 that Nadal flattened out his forehand (a little), improved serve + backhand - that he was able to win outside clay. Of course with these improvements he won 3 slams in 8 months, but then he got injured and lost momentum.
I think Nadal took a big step back when he modified his 2004 forehand. Back in 2004 all the variations of his forehands were deadly. Because the stroke had so much penetration, even if he hit it short, it was not a big deal.
Now.however, his cross court forehand is useless, unless he get GOOD depth on it. It's basically just spin with little penetration. So if he hits it short, guys like Djoker, Delpo, Murray take him to the woodshed with their backhands. Given his hollow stick, Nadal relies 100% on racquet head acceleration and arm speed to generate depth. So whenever he is tight or tired, he cannot brush the ball enough, and it become a short moonball which is obliterated by his opponent.
The only way for Nadal to attack is to use his off-forehand i.e Inside out or DTL. By their nature, these are flatter strokes (than cc fh) and penetrate through the court better. Plus Nadal has great disguise on them. The only problem is that Nadal doesn't hit them enough. And when he's not confident, he hits 95% cross court, which makes him one-dimensional and predictable.
Outside of going back to the 2004 forehand , for Nadal to counter the big hitters, he needs to use a heavier stick (with some more juice in the frame) so that he doesn't hit short and the ball penetrates the court better. That, and use the off-forehand much more. That will be the key to Nadal in the tail-end of his career in the near future..