Problem 101

(1) Solution: In $a_n = \frac{3}{4}S_n + 2$, setting $n=1$ gives $a_1=8$,

Since for any positive integer $n$, $a_n = \frac{3}{4}S_n + 2$ holds, we also have $a_{n+1} = \frac{3}{4}S_{n+1} + 2$,

Subtracting these equations gives $a_{n+1} - a_n = \frac{3}{4} a_{n+1}$,

Thus, $a_{n+1} = 4a_n$,

And since $a_1 \neq 0$,

The sequence $\{a_n\}$ is a geometric sequence,

Therefore, $a_n = 8 \cdot 4^{n-1} = 2^{2n+1}$,

Hence, $b_n = \log_2 a_n = 2n + 1$;

(2) Proof: $c_n = \frac{1}{b_n b_{n+1}} = \frac{1}{(2n+1)(2n+3)} = \frac{1}{2}\left( \frac{1}{2n+1} - \frac{1}{2n+3} \right)$

Therefore, $T_n = \frac{1}{2}\left[ \left( \frac{1}{3} - \frac{1}{5} \right) + \left( \frac{1}{5} - \frac{1}{7} \right) + \cdots + \left( \frac{1}{2n+1} - \frac{1}{2n+3} \right) \right]$

$= \frac{1}{2}\left( \frac{1}{3} - \frac{1}{2n+3} \right) = \frac{n}{3(2n+3)}$

$= \frac{1}{6} - \frac{1}{2(2n+3)}$,

Thus, $T_n$ is monotonically increasing, and when $n=1$, $T_n$ has its minimum value $\frac{1}{15}$,

Therefore, $\boxed{\frac{1}{15} \leqslant T_n < \frac{1}{6}}$.