Fix the production function to a constant value $f_0$ so that
$$
f_0 = 600k^2l^2 - k^3 l^3 \tag{1}
$$
This is a figure for different values of $f_0$ (isoquants)

Note that with increasing $k$ the rate of change of $l$ becomes smaller for a given $f$ and a fixed change in $k$. We can show this more formally by taking the derivative w.r.t $l$ at both sides
$$
0 = 600\left(2kl^2 + 2k^2 l \frac{\partial l}{\partial k}\right) - \left(3k^2l^3 + 3k^3l^2\frac{\partial l}{\partial k}\right) \tag{2}
$$
You can further simplify this to
$$
0 = -3k l (-400 + k l)\left(l + k \frac{\partial l}{\partial k}\right) \tag{3}
$$
The solutions are $k = 0$, $l = 0$, $kl = 400$ or
$$
\frac{\partial l}{\partial k} = -\frac{l}{k} \tag{4}
$$
Which confirms that that increasing $k$ decreases the MRTS, in other words, the range in which the marginal diminishes is $k > 0$