Increasing reservoir capacitance may give you the increased bass and mid bass you're looking for. Film bypass caps on electrolytic reservoir caps good for bass articulation. The other mods you've done or contemplating doing don't really change the quantity of bass.
Something to consider is you can always remove expensive coupling caps when selling or moving on from this amp, use them in another project, .47 and .22uf are common values. In my current custom build 300b mono SET's I bypassed PS caps with Duelund's silver bypass cap, Duelund CAST tin coated copper coupling caps and Texas Components TX2575 resistors in signal path. This along with top flight tubes made a huge difference in resolving/transparency capabilities. I experimented with the VCaps and Audyn True Copper Max, CAST won out. Jupiters are very nice for coupling caps as well.