Let $N$ be a prime and $\phi $ be a Hecke-Maaß cuspidal newform for the Hecke congruence subgroup $\Gamma _{0}(N)$ in $\operatorname{SL}_{n}(\mathbb{R})$ . We define a notion of the bulk $\Omega _{N}$ of the space $\Gamma _{0}(N) \backslash \operatorname{SL}_{n}(\mathbb{R})/ \operatorname{SO}(n)$ with respect to a compact set $\Omega \subset \operatorname{SL}_{n}(\mathbb{R})$ . For any prime $n$ , we prove sub-baseline bounds for the sup-norm of $\phi $ restricted to $\Omega _{N}$ . Conditionally on GRH, we generalise this result to all $n \geq 2$ . The methods involve a new reduction theory with level structure, based on generalisations of Atkin-Lehner operators.