I love to modify and "Hotrod" my amps to suit my tastes, but I also spend a fair amount of time up front with a new amp using a parametric or graphic EQ at the very end of my rig chain - (just before the amp input) and see how that works out first. By carefully adding and subtracting different combinations of frequency bands, you can gain a lot of insight, and you may work out right then and there the tonal and response changes you are looking for.
Believe me when I tell you there is much to be gained by doing that first, and some radical tonal and response changes can be had by just feeding the amp a different "frequency curve" at various output gain levels, while you back off on the amp volume and tone controls (or run them flat out, as you see fit). You need to know all this anyway if you want to craft your sound and you will probably "hone in" on the frequencies that make a difference, so I start with that because it's quick, easy, and doesn't require modifying the amp more than necessary... and it's 100% reversible in a nano-second ! Using an EQ up front works much better on older classic circuits as well I find, with more dramatic results.
Now to the amp, first thing you should realize is that speakers and cabinets are going to net very big differences in tone and to a degree overall response, so if you can find other speakers and cabinets you can experiment with, I suggest you do that next. Unfortunately, we often don't have the resources to buy multiple speakers to test, but you can buy used speakers on Ebay, and then re-sell the ones you don't like. That will cost the least for your potential "mistakes", but in the process you will still find out a lot about what works and what you really need.
The term "Punchy" bass get's thrown around a lot, but because it's a qualitative adjective for the most part, I think it needs a better description. you can add larger coupling caps just prior to the 6L6 out put tubes, and a larger cathode bypass cap, while nocking back the bass content in the preamp stage by doing the reverse, and limiting the bass on input with an EQ pedal. Looks like the 5e5a already has fairly large .1uf coupling caps before the power tubes, so the trick will be to find a speaker with a lot of bass, and nock off certain bass frequencies early on so you get what you want in the end.
I find a tightness of bass if I pull down the bass around 200-300hz, and then boost the bass back up a little below that point around 150hz, and then in the very low range, radically roll off the bass below 100hz or so, taking it down as much as possible on the input or in the amp. If you introduce very low bass anywhere in the chain (with the possible exception of the speakers themselves), you will muddy up your sound and clash with the bass guitar, and that serves no purpose, other than to sound "Big" playing in a bedroom by yourself.
You can use small .01uf coupling caps after the first preamp stage (just before the PI grids), and use a smallish 1uf cathode bypass capacitor on the V1 tube, but again I would do this after I exhausted ever trick with new speakers (adding natural bass provide by speakers) and by using an EQ up front first to filter out objectionable frequency bands.
Not trying to deter you from modifying your amp in any way, but just trying to set expectations for what I have found works best and to try first before you pick up a soldering iron. I wish someone would have told me all that before I dug in and found out the truth ! Still, everything is important, just some things more than others.