Adding a resonator is not going to help you with volume. Get a quieter axleback/catback.
I know you said you're not interested in putting the OEM muffler back on, is it because or the tips? the weight? or too quiet? Have you had the OEM in place with that aftermarket header and overpipe? It's worth a listen and easy enough to do, especially if you still have it...$0 to try.
