Can the stock muffler on a roadliner be modified to make it louder
Yes, no, maybe, not really....
The stock muff on the strat/rodie is not 'serviceable' like most other exhausts. You cannot remove baffles, gut them easily etc.
The exhaust is a welded unit and contains the baffles and a form of catalytic converter.
I have heard of some people digging in the output side and drilling holes but the end result is a mixed review in regards to sound quality.
Aftermarket slip-ons can be bought between 300 and 500. IMO, the lesser expensive ones have less expensive chrome...
Most people claim that aftermarket slip-ons only add sound but the performance is lessened. From what I have read and understand from the ones that have swapped the exhaust, the factory has given the best performance overall.
IMO, if you are not into the performance and just want a bit meaner sound, buy a quality slip-on and keep the factor muffler around in the case you get tired of your ears ringing.
Hope this helps.