I have a Deep South 2L barrel and a 3L barrel and have been playing around with this for the last couple of years or so.
My first barrel was the 2L and I did Maker's to start with. Here is the results (original on left, barrel aged at 3 months on right):
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The flavor was significantly better after being aged - much more oak and overall way better across the palate from start to finish. Actually make run of the mill Maker's a good whiskey to sip on.
Since then I've done:
Elijah Craig (6 months) - learned that aging a good whiskey that long doesn't have a whole lot of impact, so I'd probably never go past the 3-4 month mark again.
Weller Antique 107 (3-4 months) and it became a stellar drink.
Buffalo Trace (3-4 months) - much like Maker's, it made it infinitely better and increased the flavor profile.
I've also vatted a Weller Reserve and a Weller 12 in a 60/40 ratio for 4 months and 6 days that was pretty solid - way better than a normal vat in a bottle because of the aging.
Right now I have a Four Roses YL in my 2L barrel that I'll tap around Christmas or New Year's - that will put about 3 to 3.5 months of aging on it.
I also change the environment of my barrels when I have something aging - I'll keep it int he garage for a few weeks, then put it in the fridge for a couple of weeks and bounce back and forth. Sometimes I keep it on the kitchen counter for a week in between - the whole point is to try to replicate seasonal temperature changes that you'll find in a normal warehouse in KY - that makes the wood expand and contract, which is where the actual aging occurs.
It's a fun hobby that will let you play around with all kinds of different bourbons. Don't mess with the essences - they are for neutral spirits. Bourbon doesn't need it (nor would something like Rum or whiskey). Will you take a Weller 12 and turn it into a Pappy? Nope, not on your life. But you can take a run of the mill production bourbon and make it a very good sipper and enjoy the process.